This paper examines two foundational problems in early modern epistemology. The first section analyzes Descartes' famous argument "Cogito ergo sum" — the claim that the act of thinking itself proves one's existence — and presents Gassendi's objection that thinking is an arbitrary basis for such a proof. The second section explains why empirical science is central to John Locke's theory of knowledge, distinguishing between primary qualities (objective, measurable properties of objects) and secondary qualities (subjective perceptual effects on the observer). Together, the two sections contrast rationalist and empiricist approaches to establishing certain knowledge.
The paper demonstrates the technique of philosophical objection and rebuttal. When presenting Gassendi's challenge, the writer does not simply report the objection but responds to it — pointing out that inanimate objects can walk without possessing genuine existence, which restores force to Descartes' original argument. This move shows how to engage critically with competing positions rather than merely cataloguing them.
The paper is divided into two distinct question-and-answer blocks. The first covers Descartes' Cogito (roughly three paragraphs: statement of the argument, the method of doubt, and Gassendi's objection with rebuttal). The second covers Locke (two paragraphs: the role of empirical science, then the distinction between primary and secondary qualities with examples). Each block moves from general principle to specific illustration.
Descartes' "Cogito ergo sum" reads: first, I think, and then, therefore, I am. In other words, I know I exist and I am certain of my own existence because I have the capacity to form thoughts, including the thought of doubt. For me to articulate "I think" — however accurately or inaccurately the thinker may be observing the nature of reality, whether the thinker is lucid or whether the thinker is dreaming — there must be an "I" doing the thinking. Therefore, "I am" is concrete evidence of existence. "I" know I have the ability to think, and to think the thought that "I" exist; therefore "I" must have an existence.
Before irrefutably (in his own estimation) establishing the veracity of his existence, Descartes begins his famous meditation by deciding to doubt all things. He realizes, however, that the one thing he cannot doubt is that there must be someone doing the thinking and the doubting. He thus cannot doubt his own existence. Therefore, I think, therefore I am — for there must be someone "there" even to begin the process of thought.
Gassendi argued that Descartes' assertion was not valid because, by the same logic, one could say that human existence is not a dream because "I walk, therefore I walk, therefore I am." In other words, using thinking as the sole substantiation for existence is somewhat arbitrary. Descartes could have selected any property of humanity — or of physical matter in general — as a justification for existence and as a denial of the possibility that one must doubt everything, including one's own existence.
Yet although the mind, as in a dream, can be deceived as to whether the body is walking, there still must be an "I" doing the thinking, dreaming, or hallucinating. Furthermore, inanimate objects can "walk" — a doll or a puppet can walk, provided there is someone manipulating its physical motions — but the existence of these objects can be in doubt, since they might be delusions or hallucinations. Thinking, therefore, retains a privileged status that mere physical action does not.
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