The other kind is what the author calls 'theoretical' knowledge...

The other kind is what the author calls 'theoretical' knowledge, whose truth cannot be established except in the light of the first kind. Among the examples given are: 'The earth is spherical', 'Heat is caused by motion', 'Infinite regress is impossible', 'The angles of a triangle are equal to two right angles'. The author does not seem to be right here in putting two different kinds of statements in one class called 'theoretical knowledge'.

'The earth is spherical' is not the same kind of judgement as 'The angles of a triangle are equal to two right angles'. The former requires observation and inference for its proof, while the latter can be established by pure reasoning. The same distinction applies to the two statements 'Heat is caused by motion' and 'Infinite regress is impossible'. All knowledge is based on previous knowledge, which in turn depends on knowledge preceding it.

The a priori or primary knowledge is that irreducible remainder which does not arise from any previous knowledge. A part of primary knowledge, consisting of such general principles as the law of contradiction, constitutes the basic condition of all knowledge. Without it no general proposition can be affirmed. It is this knowledge independent of experience that makes metaphysics possible. The progression of thought is from universal to more particular propositions.

This is true even in the experimental sciences, which cannot dispense with the universal principles of causality and uniformity of nature. Experimentation also, without the application of necessary rational laws, does not lead to general scientific truths. The Islamic philosophers, including al-Sadr, espouse this theory. According to the empiricists sense experience is the primary source of all knowledge. They do not admit the existence of any necessary rational knowledge prior to experience.

There can be no knowledge of universal truths prior to experience. Their position makes metaphysics and deduction impossible. The empirical doctrine has to be rejected for the following four reasons. First, either the empirical doctrine is prior to experience or it is not. If it is, it refutes itself. If it is derived from experience, the validity of experience as a criterion of knowledge has not yet been established.

Second, empiricism fails to affirm the existence of matter and the external world, which cannot be affirmed except by primary rational knowledge.