In terms of metaphysics, Descartes holds to very specific principles, all of which support his conviction that doubt is the only sure means of understanding truth and reality. The Cartesian concept of identifying existence as such relies on the philosopher’s conviction that dualism is the defining aspect of human existence. The mind and the physical body are distinct substances, and Descartes supports this by the logical course of noting how the body cannot think, just as the mind is subject to the sensory impressions provided by the body. In plain terms, the mind is a thinking thing, the body is not, so the two are in a sense mutually exclusive. Controversially, in fact, Descartes’s Meditations express that the mind may exist independently from the body. This generates intense argument; how, it is asked, may a mind function in any way apart from the body, when the physical brain is essential?

The answer lies in the metaphysical quality of the Cartesian concepts. More exactly, Descartes does not insist that the mind in fact does exist independently from the body; rather, he asserts that this is a possibility, given the natures of the substances as so fundamentally different. Then, this possibility in itself relies on another Cartesian concept subject to dispute, in that Descartes, having established that his ability to doubt and reason is proof of his existence, also perceives the ability as confirmation of the human mind and soul as created by God. This being the Cartesian reality, it then follows that God, capable of anything, could allow for the human consciousness as separate from the body. Nonetheless, Descartes’s separation of the substances is also based on rationality and science. Most importantly, he insists that human beings often and mistakenly attribute mental activity to that which has no mind, and also ignore that the body functions in many ways removed from conscious thought. For the philosopher, there are certainly interactions common, if not inevitable, between mind and body, as physical behaviours reflect mind/body connections. At the same time, however, Descartes still asserts that it is vital to understand that these dual substances are inherently distinct from one another, which in turn enhances the human ability to employ doubt and reason as the way to comprehending existence. In the past and today, there is no lack of criticism leveled at Descartes’s thinking but, when his processes of his own reasoning are examined, there is logic in place.

Similarly, John Locke very much relies on logical progressions to create a philosophical framework. A great deal of Locke’s work is more pragmatic than that of most philosophers; he addresses concrete concerns of finance, government, and social structures, for example. At the same time, his epistemology in general remains rooted in ideas going to the most fundamental realities of human existence, and this is evident in his thinking on how human ideas themselves are created. For Locke, these take forms of the simple and complex, which concepts also relate to his other perception, as will be seen. He begins by asserting how, logically, there may be no ideas whatsoever in the human being until actual experience provides them; the infant mind is indeed a blank slate. Sensory input is then the provider of how simple ideas are first generated. A child, for example, gains the idea that an unpleasant feeling of hunger is eliminated when they take in food.
Complex ideas come into play when various simple ideas are combined in relational ways; the human mind is then reaching conclusions through comparing and utilizing multiple impressions.

If eating satisfies the child’s hunger, for example, and remains as a simple idea, this is expended when the child has further experience of eating and discovers that certain foods have disagreeable effects. In Locke, this process of arriving at complex ideas through combining the simple is a mode, and there are virtually no limits as to how human minds may build upon ideas, and also apply subjective reasoning to reach what are believed to be truths. The relational process here is as well dependent upon the natures of what human beings experience and perceive, and this is explained by Locke’s distinction between primary and secondary qualities. No idea may be created without the causal impact of a quality or property, but these take two essential forms. The primary is that which is removed from any form of subjectivity; it exists apart from how, or if, it is perceived by anyone. A table’s size and weight, for example, are immutable properties, no matter their being recognized as such.

When, however, the table is seen by an individual and the darkness of the wood is acknowledged by the person, what occurs is a secondary impression because the quality of darkness does not exist in a fixed way in the external world. Locke asserts that the same is true of colour itself which, while frequently taken as a defining quality, is innately subjective. If the table is of brown wood, this “brownness” is real only insofar as it is perceived by whomever is experiencing the table. This then supports Locke’s thinking on representational realism. A thing is real in that it has certain properties of a primary nature, but human beings invariably attach ideas to it based upon how they perceive elements of it. Essentially, it is the human experience of the thing, rather than the thing itself, that promotes ideas as to its identity. Strongly implied in this thinking of Locke’s is that, as the human being develops more complex ideas, the subjectivity in assessing the realities of external objects becomes more pronounced, and qualities by no means “real” take on the quality of the real.