Descartes wants certainty. The Cartesian theory is the fruit of an attempt "to establish a firm and abiding superstructure in the sciences". At first this may not appear the objective since he starts by doubting everything. The rationale behind this general doubt is that it frees the seeker from prejudice and allows him to disengage from the senses, the senses being unreliable and hence lacking certitude. Unfortunately the theory accepts particular premises without examining the certainty they themselves possess. The particular ideas are open to doubt. Basic elements to the theory, their doubtfulness leads to a collapse of the whole thing.
One may legitimately reject the whole of something which is only in part doubtful. Descartes asserts this. Granted doubt does not disprove what is in question, but lack of proof of its validity causes the doubt in the first place, and amidst skepticism it is precisely this assurance one needs to build a stable framework for reality.
What could this first necessity be? Nothing perceived as external the mind, since these ideas may actually be whimsically self-generated with no bearing to reality. What about thoughts recognized as generated by the self? Better yet, what about the self? The self, the thinker, the doubter cannot be doubted. To be skeptical of one's own existence is to reaffirm it. To doubt there must be a doubter. The Cartesian theory has at least one certainty then, but what of it? Realizing one's reality by itself fails to shed any light on any other phenomena.
The idea of the thinker's existence is clearly apprehended, and all ideas so distinctly perceived are taken by Descartes as true. He insists what is clearly perceived in some ideas (e.g., the earth) is not the appearance they have, but that "the thoughts of those objects were presented to my mind". He raises the possibility that a malevolent god may be responsible for this; the certainty may itself be illusory. The concept of god, however, precludes this. God is the world's ultimate cause. The ultimate cause must possess the ultimate power, which means god is infinitely wise, eternal etc. et al. God is infinitely good; malevolence and deceit reflect a deficiency in the ability of one to execute one's will (Descartes does not seem bothered with the possibility that a malevolent god, capable of causing mortals to err where things seem the most certain, could cause mortals to think malevolence is a deficiency). With the possibility of a god, it becomes necessary to examine which ideas originate from the thinker and which from the deity. If the thinker discovers all his thoughts may be his own, then it is just as likely god does not exist as it may exist. If he finds ideas that are clear and not his own, then they must come from another being. This being is God.
What sort of ideas are imposed on the thinker? Perceptions such as color, pain, and heat are fuzzy and easily accounted for the the thinker's mind. Substance, duration, and number are also possibly ideas of internal origin.
I think of myself as now existing, and recollect besides that I
existed some time ago, and when I am conscious of various thoughts
whose number I know, I then acquire the ideas of duration and number.
The idea of God seems so unlike other ideas. Examining the notion, one finds God to mean "... a substance infinite (eternal, immutable), independent, all-knowing, all powerful... by which I myself... were created". Modest Descartes, reflecting on these excellent properties, cannot believe they originate from himself, a finite creature, and so concludes the idea must come from an existent god.
for though the idea of substance be in my mind owing to this, that I
myself am a substance, I should not, however, have the idea of an infinite
substance, seeing I am a finite being, unless it were given me by some
substance in reality infinite.
The concept of infinite powers is inseparable from that of God. Descartes claims it comes from the infinite being itself, but there is a problem with that postulate. He readily admits the concept of duration may develop wholly from the thinker's mind. Something eternal exists for all time, but if the concept of time is a product of how the thinker arranges other thoughts, there is no independent reality to time. Big blocks of time like eternity may be extrapolations; products of a will trying to operate beyond the realm of its understanding. Concepts such as omniscience may have a similar basis. Indeed, the whole idea of infinity is hardly clear at all; if it exists it must transcend all things. A protestation to the effect that the idea of infinity can be clearly perceived but not imagined (i.e., visualized as an imagined triangle is visualized) is insufficient. Were it to suffice, then a doubter may clearly perceive a square circle.
In the spirit of general doubt, it is not possible to accept the Cartesian proofs for a god. God's attributes are open to doubt. The demand for an ultimate cause, an ultimate mover, rests on the concept of cause and effect (i.e., where there is an effect there is a cause). This is also open to doubt, since it rests on the idea of chronology which in turn depends on how the doubter's mind arranges thoughts. The rest of the theory depends on the existence of a benevolent deity. Without the god it collapses down to its very foundation, the skeptical mind. Does the doubter exist? If nothing else, the attempt of the doubter to deny himself confirms that he exists so that he may try. Actually, the concept of cause and effect is neither proved nor disproved. An irrepressible faith in one's existence is all that remains. This can account for the search for Truth. After all, what does one's conduct matter if there is no soul to be rewarded or punished in the present or by a god in some indefinite future? What does it matter if there is not being to act? Do I really exist? "I" never seriously doubts it.