Tom asked:

What kind of knowledge (a priori or a posteriori) did Descartes discover in the cogito argument, and how much help is it in defeating skepticism? What more is required in order to have knowledge of the real world?

Answer by Martin Jenkins

In his Meditations, Descartes employs a process of rigorously doubting what he believes and is convinced he knows. He does this in the hope of discovering an indubitable, certain truth which can act as an unshakable epistemological foundation for human knowledge and for the developing Natural Philosophy (Sciences) of his time. In this quest, anything which even hints of doubt is rejected.

Knowledge acquired by the senses is judged as unreliable. A convincing distinction between waking and dreaming is not found. Even the ‘certainties’ of arithmetic and geometry could be deceptions. The existence of a benevolent God who upholds truth is questioned for perhaps there is instead, a malevolent deceiver. Rather like in the Matrix films, the deceiver maintains deception whereby all that is experienced as real could be unreal.

At the end of Meditation I, Descartes concludes that he cannot know anything with the certainty he seeks.


In this intellectual abyss, Descartes questions the truth of his own existence.

Yet if he is deceived then something must exist to be deceived. Further, this something is a thinking thing – Descartes has evidenced this by the very process of doubt he has employed.

This thing is a thinking thing which reflects, which doubts. It exists because it thinks. Although the existence of his body cannot be proven, that of the Thinking thing or Cogito can.

The Cogito as Thinking thing qua thinking, therefore exists and this cannot be doubted without engendering absurdity. This evinces a self-evident Truth, a certain truth which is the foundation Descartes had been searching for. It is a contradiction to state I am thinking but I don’t exist as the latter would remove the conditions of possibility for the former. The status of this truth is apodictic – it is self-evident when analysed and the opposite is impossible.

In the remainder of Meditation II, Descartes reflects upon how he arrived at such a self-evidently true conclusion as the means employed might assist in the discovery of more truths.

Reflecting on the wax before him, he notes that although there are inessential qualities which change, appear and disappear (such as its taste, smell, texture) ; there are also essential qualities such as its occupation of space, its motion and flexibility. These qualities cannot be doubted as existing.

How did he arise at this conclusion? The conclusion is derived by the faculty of Judgement employing and dealing with what Descartes called Clear and Distinct Ideas. These possess an Objective reality. In other words which Descartes doesn’t use, clear and distinct ideas display sound a-priori reasoning which cannot be doubted, challenged on pain of committing contradiction. The reasoning involved leading to a conclusion which is sound, or where the conclusion follows from the premises, is as already said, apodictic. As you put in your question Tom, this is the knowledge Descartes discovers in his meditations on the Cogito.

With respect to Descartes reasoning, if the sceptic doubts the internal soundness of a-priori reasoning, then the sceptic cannot understand or appreciate ‘logical’ thinking. The sceptic could however, recognise the internal soundness of a-priori reasoning but question whether its conclusive truths are applicable to ‘the world’ beyond the reasoning process…

Of the World: That it Exists

Returning to the Objective reality Clear and Distinct Ideas display (a-priori reasoning) , Descartes further wonders where it comes from – for no thing can arise from nothing.

Importantly, one idea bridges the certainty of the Cogito with the certainty of the existence of an independently existing world and of our knowledge of it- this idea is that of God. In Meditation III, Descartes argues that the idea of infinity-which he attribute to the nature of God – cannot arise with a finite creature such as himself, so it must arise from something other – God. Secondly, Descartes reasons that he cannot create himself and sustain himself in existence so, this also must arise from something else – God. (Variant on the Causal/Contingency Argument for God’s existence here) Thirdly, as no thing can come from nothing (Nihil ex nihilo), there must be a first cause before all other effects -including the cause of Descartes’ own existence. What is in the cause must to some degree or other, be in the effect (Aristotle’s account of Cause here I think). As Descartes has concluded that he is an unextended, thinking thing which exists then, he must, as an effect, contain to some degree, characteristics of that which caused him or created him. That cause is God: an eternal, infinite, thinking thing.

Note here Tom, the employment of clear and distinct reasoning — that there can be no effect without a prior cause, that the finite cannot create the infinite and so on. For rationalist philosophers, this method is indubitable and yields sound conclusions about the nature of reality – ontological conclusions. As Descartes believes he is created by God and that what is in the cause is also in the effect, then God as the supreme thinking thing, has also placed the faculty of Understanding, of the recognition and use of clear and distinct ideas, in his thinking creations (Meditation IV). Mistakes, illusions arise in finite minds from the bad application of the Free Will or Judgement in matters of the Understanding. As finite creations, such mistaken thinking is to be expected. However, Descartes believes that this mechanism of thinking will, can and does provide certain knowledge of the world – if correctly executed. It further lays the grounding for Natural Philosophy’s attempts to understand the world by mathematics and geometry.

In Meditation V, Descartes yields the knock out punch by employing the Ontological Proof for God’s existence. Again, it is achieved by employing clear and distinct ideas which exude objective reality. If one can understand the nature of God as all-powerful then, one must conclude that God must necessarily exist. If he didn’t, he wouldn’t be all powerful – which is a contradiction. God’s existence has to be conceived along with his nature just as the conception of a mountain involves that of a valley. So God necessarily exists. As God is all powerful, he is perfect. As perfect he cannot deceive. So as creator of all reality, God could

not deceive Descartes that the world he perceives and more importantly, conceives by clear and distinct reasoning does not exist. So the world necessarily exists. Hence Descartes has acquired knowledge of a real, independently existing world beyond his senses.