"Deus sive natura"

("Whatever is, is in God or nature.")

It follows then that this "substance" or "God" must be indivisible and completely unified, as well as eternal.

For Spinoza, the appearance of separateness that we see around us is not an accurate representation of the underlying reality. These separate phenomena are all aspects of a single "substance" or "God." The two dominant attributes of this "substance" or "God" are "extension" and "thought." The universe that we perceive around us, with its diverse physical phenomena, is a part of God’s essence. This is what Spinoza is referring to when he speaks of "God’s" or "nature’s" "extension." In the mental realm, the existence of thought – modified by "infinite intellect" – produces the "truth." This "truth" includes all of the mental events which are the modes of "thought."

Spinoza went on to argue that the realm of "extension" and the realm of "thought" were causally independent of each other. Each of these realms were closed and self-contained systems. Despite the impossibility of any causal interaction between them, Spinoza speculated that the inevitable unfolding of their independent attributes must proceed in parallel with each other. According to Spinoza, "The order and connection of ideas is the same as the order and connection of things." As such, every physical event in the world of "extension" must have a corresponding mental event in the world of "thought." This is so because everything flows from the same infinite being.

What does this mean for us as human beings? According to Spinoza, we are not substances, for only Nature or God is truly substance. But then what are we? Because our existence is reliant upon the reality of the one real "substance", we must conclude that we are then but a small part of that substance. According to Spinoza, we exist as "modes." As "modes", what kind of knowledge can human beings attain? Spinoza outlined three distinct forms of knowledge:

Opinion
The first type of knowledge is "opinion." This is the most unreliable source of knowledge, for it depends either on our sensory experiences, or else upon our memory and imagination. Therefore, we should disregard the misleading information provided by our senses, as well as the customs and habits into which we have been indoctrinated.

Reason
The second type of knowledge is "reason." By analyzing the effects of particular phenomena, we can reason back to original causes. This will lead us back, eventually, to the ultimate cause: the final truth.

Intuition
The third type of knowledge is "intuition." By using our "reason" to recognize the original cause, we then use this knowledge of the "divine essence" to intuit everything about reality: what was, what is, and what it will be.

Spinoza used the above argument as a foundation for morality. But how can we live a good and "moral" life if we are unaware of how our actions affect the essence of reality? What we define as "good" may in fact, over the long term, be "evil." Therefore, the greatest good that humans can do is to understand their place within the structure of the universe, this universe being a natural expression of the essence of "God" or "Nature."

But if everything is determined, as Spinoza maintained, how can we then speak of good or evil? If our actions are predetermined strictly by cause and effect, how then can we speak of human freedom and choice? By acquiring an adequate knowledge of the desires and emotions that are the "causes" of my "effects", I become free. This freedom is my reward, because it allows me to see that I am a significant "mode" in the greater reality.

28. Gottfried Wilhelm Leibniz (1646-1716)
Leibniz was a mathematician, as well as a philosopher. His mathematical achievements were extensive: He invented infinitesimal calculus. But it was for his contribution to philosophy that Leibniz is most recognized. Leibniz published only three books on philosophy during his lifetime. These were Discours de metaphysique (Discourse on Metaphysics) in 1686, Théodicée (Theodicy) published in 1710, and La Monadologie (Monadology) published in 1714. These books were not overtly technical, and were written for the general reading public. The more technical aspects of his work were discovered, centuries after his death, in notebooks and letters.

In the field of philosophy, he was the first to distinguish between "truths of fact" and "truths of reason." In so doing, he sought to draw a distinction between the "contingent" world of observation, with that of the logical or empirical world of reason. By applying rigorous and formal reasoning, Leibniz believed that we could attain knowledge of the ultimate structure of reality. The technique that Leibniz used was logical analysis. For Leibniz, every proposition can be expressed in the form of a subject-predicate. Additionally, every correct proposition is a statement of identity: that is that the predicate is wholly contained in its subject. For example:

2 + 3 = 5

According to Leibniz, everything that we know or believe can be expressed in one of two forms: "truths of fact" and "truths of reason." One of the most significant aspects of his philosophy was that all existential propositions are "truths of fact" and not "truths of reason."

For Leibniz, the "subject" of any proposition signifies a single, self-contained, and indivisible "object" or "monad", while the "predicate" expresses a variable such as "quality" or "property." Unlike Descartes, whose system was dualistic, and Spinoza, whose system was monistic, Leibniz proposed a system based upon plurality. Within this plurality, there existed an infinite number of "monads." These "monads" were unique unto themselves, and each experienced the universe from a unique perspective. This view was in stark contrast to Spinoza, who viewed the universe as a single substance.

29. Isaac Newton (1642-1727)
The true founders of the period known as the Enlightenment were John Locke and Isaac Newton. It was Newton - the successor to Copernicus and Galileo - who wrote one of the most significant and influential books of the time. This was the Philosophiae Naturalis Principia Mathematica or The Mathematical Principles of Natural Philosophy, which he completed in 1687. This publication sought to create, by applying the principles of logic and mathematics, the first great synthesis of mathematics with nature. By utilizing many of the ideas found within the work of Spinoza, Leibniz, and Descartes, Newton attempted to apply these new principles to the natural world. Newton’s own work had a revolutionary impact in the 18th century. Because of him, there was a renewed interest in applying the tools of logic and mathematics to the natural world. In these, the 17th and 18th centuries, natural science gained prominence over purely abstract mathematical reasoning as a means of understanding reality. Along with Newton’s insights into the laws of motion and gravitation, there were other breakthroughs among his contemporaries in chemistry, physics, and biology.

30. John Locke (1632-1704)
John Locke was greatly impressed with the achievements that were being made in the natural sciences, and wished to extend these achievements by creating a complete theory of knowledge. This theory of knowledge would be based on a careful and rigorous study of nature. His goal was to establish a comprehensive epistemological foundation for knowledge - one that was devoid of superstitions or uncritical assumptions.

Locke’s first question was "How to we acquire knowledge?" In his An Essay Concerning Human Understanding (1690), he began by studying what our knowledge consists of. It is made up of ideas. These ideas can take many forms. They encompass perceptions, emotions and reflections. Locke’s primary assumption about ideas was that they were not innate. They were derived solely from our experience. Locke proposed the principle of empiricism, which states that all our knowledge is derived from experience. This "experience" is obtained either by sensation (from the external world) or by reflection (from the inner world of the mind). Anticipating the eventual emergence of linguistic philosophy, Locke showed the relationship of words to ideas. For Locke, words signify ideas. Disputes arise when words are used carelessly, without clear "signification." Therefore, the "word" and the "idea" can only be meaningful if both co-exist. To use a "word" without reference to a particular "idea" is meaningless. There was also on additional problem: The Extent of Knowledge. Locke understood that our knowledge is quite limited. If we can only achieve knowledge when we have a genuine understanding of the "essence" of things, then it follows that our knowledge will always be limited. Locke had to finally conclude that achieving a completely epistemological foundation for knowledge was beyond our reach. We must content ourselves with relative or probable knowledge. But this, according to Locke, was sufficient for our purposes. We have, within our grasp, access to knowledge which will secure our "great concernments." These "great concernments" relate to our survival in everyday life. We do not need a great understanding of the "essence" of food, for example, in order to survive. We can exist and prosper, even with partial knowledge. Regarding morality and our relation to a creator, Locke held that our partial knowledge of things was sufficient for our needs.

31. George Berkeley (1685-1753)
A major critic of Locke’s work was George Berkeley. Berkeley believed that Locke had not carried the principles of empiricism to their logical conclusion. He proposed an entirely radical alternative to the moderated empiricism of both Locke and Descartes.

Berkeley held that these earlier philosophers had failed to draw the correct conclusion from their studies. They had tried to avoid the problem of attaining knowledge by the act of separating "material" objects from our ability to perceive their "essences" correctly. In fact, this "representationalist" model of knowledge acquisition, according to Berkeley, was inherently flawed.

Berkeley countered that there was a viable alternative. Our common sense clearly shows that perception consists of two distinct elements: the "perceiver" and the "perceived." The "perceived" can only be ideas, since these are the only things that are real. We must discard the notion that material objects are real since we cannot confirm their existence - in a truly empirical sense – simply by the act of observation. We continually encounter the problem of "relative perception" or "relative essences."

In fact, the model put forward by Locke changes from:

Perceiver - Ideas - Material Objects

To Berkeley’s model of…

Perceiver - Ideas

For Berkeley, only the "ideas" are real. The "representationalist" position is flawed in two major respects. The first is that it cannot provide a purely empiricistic account of the connection between "ideas" and the "objects" they are meant to represent. Secondly, this flawed empiricism has serious consequences. It creates a relativistic perception of reality. This relativism inevitably leads to skepticism, and hence to atheism. To avoid this relativism, we must embrace immaterialism.

For Berkeley, there are no "abstract" ideas. In his Principles of Human Knowledge, Berkeley argued strongly against Locke’s assumption that "general terms" (or words) can signify "abstract" ideas. There are no abstract ideas. Everything exists within the mind.

If we accept Berkeley’s claim that materialism leads to atheism, then the converse is also true: that immaterialism can restore our religious faith. Since our knowledge and "perception" of reality is "mind-dependent", then it follows that there must be a primary perceiver, God, in whose mind all ideas are contained.

32. David Hume (1711-1776)
The Scottish philosopher, David Hume, attempted to counter Berkeley’s immaterialist philosophy by extending and developing many of Locke’s philosophical assumptions. By the late 18th century, science had made incredible progress. Mere metaphysical speculation, according to Hume, had become outdated. Hume rejected the notion that there should be a positive outcome for our philosophical investigations. Instead, he felt that an extreme philosophical skepticism was necessary in order to find the truth of reality. Hume’s starting position was, however, value independent. It wished neither to arrive at a purely materialist conclusion, nor an immaterialistic one. Hume believed that the correct goal of philosophy should be to explain why we believe what we do. He explored this goal extensively in both his Treatise of Human Nature and in An Enquiry into Human Understanding.

Hume began his exploration of human belief by distinguishing between "impressions" and "ideas." "Impressions" are our "immediate" experiences. We "see" or "taste" or "touch" in an immediate sense. Our "ideas" are different. They are but flawed copies of our original experiences. They are our memories of these sensations. Hume thus declared that our primary aim should be to find the origins of these "ideas." In doing so, he distinguished between two particular concepts: "Relation of Ideas" and "Matters of Fact." "Relation of Ideas" refers to beliefs or opinions that are partially or entirely created within the mind. These ideas are contingent on speculation and theorizing. "Matters of Fact", however, are beliefs that we hold about the outside world that clearly refer to an external object or phenomena. These latter perceptions of reality are a priori beliefs. They "precede" experience, and would exist independently of our knowledge of them.

Hume gradually eroded away many of the assumptions we take for granted. Many of our beliefs are based on habit. We are conditioned to believe and accept assumptions that have no basis in reality. These habits or customs are self-reinforcing and create a "subjective reality" that we mistake for an "objective reality." Even our assumptions regarding cause and effect are an illusion. These causes and effects are distinct and separate. That they occur in "constant conjunction" is no empirically reliable guide to the validity of their existence. Even the existence and autonomy of a "self" comes under attack. Hume asks, "From what antecedent impression does the idea of the self arise?" But where, according to Hume, is the "I" that I refer to when I say, "I believe…"? No matter how hard I try to observe the events "in my mind", where is the "I" that I speak of? When I look inward, all I see are a numerous succession of individual ideas or sensations, each of which are linked by the habit of association and familiarity. Thus, our belief that there is an "I" within our minds is an illusion that we accept as a matter of habit, but not by empirical observation and verification. The idea of the "self" or the "soul" is thus an illusion. We are but a mixture of distinct and separate sensations and perceptions. Yet another illusion we possess, according to Hume, is our faith in the existence of the external world. We believe in it, not because it can be shown to exist empirically, but because of the habit of familiarity. It is natural to believe in its existence, but its provability is in question. What then can we know? It would seem that Hume’s empiricism, unlike that of even Locke or Berkeley, leads to total and absolute skepticism. Hume suggested that we adopt a "mitigated skepticism", one that accepts the limits of human knowledge. Pure mathematics because it rests entirely upon the relations of ideas – and that presumes nothing about the external world – is our only safe guide to truth.

But how do we address the question of God, of morality, or the foundation for ethical behavior? Though it is impossible to account for our feelings in a purely empirical way, our feelings can nevertheless provide a reasonable guide for our behavior. When we help someone, we tend to feel good about ourselves. When we do wrong, we have feelings of regret or guilt. Initially, this foundation for moral behavior seems extremely flawed. The subjective nature of its premises can lead just as easily to negative behavior. Hume responds by saying that our subjective approach to morality is at least equal to our faith in the existence of "facts" concerning the natural world. Both rely on an absence of rational evidence. Hume concluded that he had provided humankind with a moral principle that was consistent and no less significant than the "certainties" of the natural sciences.

33. George Wilhelm Hegel (1770-1831)
Of all the German Idealists, Hegel was the most prominent. His aim was to construct an entirely comprehensive system of knowledge about reality. Hegel held that reality must be rational. The multiplicity of things is an illusion, for they are all part of a singularity or unity. This "absolute consciousness" contains everything that is knowable, or that can be experienced. Hegel focused on the study of logic in order to frame his study of reality. By utilizing logic, Hegel proposed the concept of the dialectic. For Hegel, a concept (thesis) can be countered with an objection (antithesis), thus leading to an accommodation between the two (synthesis). This synthesis wasn’t static. It becomes the new thesis. As such, it anticipates the emergence of yet another antithesis – leading to a new synthesis. For Hegel, all of our ideas are part of a totality which he called "The Absolute" or "Absolute Spirit." In his Phenomenology of Spirit, Hegel described "Absolute Spirit" as the evolution of Spirit from Subjective Spirit to Objective Spirit, and finally then to Absolute Spirit. This final stage in our understanding - Absolute Spirit – finally transcends the old dichotomy between empiricism and rationalism. For Hegel, all history is moving towards Absolute Spirit.

34. Soren Kierkegaard (1813-1855)
One of the most outspoken critics of Hegel’s extreme rationalism was Soren Kierkegaard, the Danish philosopher. Kierkegaard was unique in the history of philosophy in that he published much of his work under different names, or even posthumously. Kierkegaard did so because he was deeply concerned with the problem of attachment. We become t