Descartes accepts the validity of innate rational knowledge.

Descartes accepts the validity of innate rational knowledge. Locke (1632-1704) is the founder of modern empiricism. While he claims that all knowledge is derived from experience there being no innate ideas or principles he divides knowledge into three types: (1) by intuition, (2) by rational demonstration, (3) by sensation. Our knowledge of our own existence is intuitive, our knowledge of God's existence is demonstrative, and our knowledge of things present to sense is sensitive.

This division of knowledge into three groups is inconsistent with his empirical doctrine. Locke makes a distinction between what he calls primary and secondary qualities. The primary ones are in separable from bodies, such as solidity, extension, figure, motion or rest, and number. The secondary qualities are only in the percipient, such as colour, sound, smell, etc.

Since there is no way, according to Locke, of knowing the primary qualities except through the senses, this division is also inconsistent with his empirical doctrine. The Idealists: The Platonic theory of Ideas, generally called 'realism', is referred to as 'idealism' by the author. Whatever we may call it, it did not involve any denial or doubt about reality. In metaphysics, idealism is the theory that reality is of the nature of mind or idea.

To al-Sadr, it is an attempt to shake the foundations of objective reality and to exterminate certainty. In order to study the role of idealism in the theory of knowledge, he proposes to examine three tendencies in idealism. These he calls 'philosophical', 'physical' and 'physiological'. Philosophical Idealism: Its founder was Berkeley, who declared, 'To exist is to know or to be known'. He denies existence to objective realities existing independent of minds. Mind and its ideas exist.

All we know of 'matter' are the qualities of our sense (the secondary qualities of Locke). Berkeley's idealism has been interpreted differently and al-Sadr has selected an interpretation that he considers best-known. He cites Berkeley's proofs in support of his doctrines. The first one is intended to prove that all knowledge is based on and comes from the senses.

The main criticism against Berkeley is that he takes for granted the law of contradiction in his proofs while denying that there is any knowledge not rooted in sense experience. The author interprets Berkeley as denying the independent existence of things and offers reasons for rejecting this alleged denial of Berkeley.