Kant’s critique of Descartes’ theory of knowledge

Adan asked:

What would Kant think about Descartes’ theory of knowledge?

Answer by Geoffrey Klempner

We don’t need to speculate about what Kant would think about Descartes’ theory of knowledge because Kant spells out his disagreement with Descartes in the Critique of Pure Reason.

Descartes believed that it was possible to intuit certain metaphysical truths because we have ‘clear and distinct’ ideas of them. But this is only on the condition, as he acknowledges, that God exists – which he thinks he can prove. If all my experience were produced by an evil demon then no ideas are clear or distinct even if we think they are. If God exists, then provided we use our powers of judgement responsibly, we can rely on our capacity to discover truths about the external world.

Descartes believes that he has a ‘clear and distinct’ idea of his soul qua ‘immaterial substance’. Belief in the existence of body as ‘material substance’ is justified because he experiences bodies outside him, in addition to his own material body, and God is no deceiver.

Kant has responses to all these points. In the ‘Refutation of Idealism’ in the Second Edition of the Critique of Pure Reason, he argues that experience is only possible on the condition that it takes a certain form, viz. spatio-temporal. We would be unable to identify ourselves as a subject if all we experienced was a series of experiences arranged in time.

A lot has been written about this remarkable argument, as an example of what has come to be known as a ‘transcendental argument’. Very good accounts can be found in P.F. Strawson The Bounds of Sense (1966) and C. Peacocke Holistic Explanation (1979).

In the ‘Paralogisms of Transcendental Psychology’ in the second part of the Critique of Pure Reason, Kant argues that knowledge of my identity through time as an immaterial substance is illusory. There would be no way to tell the difference between a continuing ‘soul’ and a series of momentary ‘souls’ communicating their states to one another like a line of colliding pool balls. In his terms, we mistake the ‘a priori unity of apperception’ for the perception of unity. Self-identity is a necessary theoretical parameter in deciphering experience, not something we actually experience.

For Kant, all we have is experience taking a spatio-temporal form, from which we form judgements about objects in space and our own position as an observer relative to those objects.

But now comes the crunch: there is no way to prove the existence of God, as Kant claims in his critique of the Cosmological, Teleological and Ontological arguments. All we can say (some commentators would say this is already saying too much) is that in addition to the world of phenomena, of which we can have knowledge in the ways described, there is a noumenal world, beyond space and time, strictly inconceivable to the human mind. If God exists, then that would be a fact about the noumenal world. Descartes’ claim that reality consists of immaterial and material substance – souls and bodies – goes beyond anything that human beings could ever know.

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