Modern skepticism has its progenitor in the post-Aristotelian Greek school of skepticism headed by Pyrrho (b.c. 360 B.C.). It did not confine itself to showing the contradictions of sense perception but went on to an analysis of knowledge to assert the impossibility of certainty. Hume took Locke's and Berkeley's empiricism to its logical conclusion by throwing doubt on causality and induction and abolished the distinction between rational belief and credulity. Not only God but also the self, other minds and external reality fell prey to a skepticism based on the denial of the principle of causality, which was again based on the empirical theory of knowledge. Hume's explanation of causality, as pointed out before, is unsuccessful.
Relativism, in the context of metaphysics and epistemology, is, according to al-Sadr, a doctrine which asserts the existence of independent reality and the possibility of knowledge, but a relative knowledge that is not free from subjective attachments. Hence the author proposes to discuss certain main relativistic tendencies, beginning with Kant's philosophy.
Kant believes that propositions are of two kinds: analytic and synthetic. An analytic proposition is one in which the predicate is part of the subject; for instance, 'The triangle is three-sided'. The synthetic propositions are those which are not analytic; they give new information.
Propositions are also distinguishable into two other kinds: a priori and empirical. A priori judgements, though they may be elicited by experience, have a basis other than experience, unlike empirical judgements which are rooted in experience. Some a priori judgements are synthetic. All the propositions of pure mathematics are a priori in this sense. The propositions of sciences are synthetic and empirical. Kant believes space and time to be formal attributes of the perceiving subject which give a special and temporal structure to all experience. He agrees with Berkeley that matter is not given in knowledge and sense experience, but disagrees with him in holding that external reality cannot exist independently of mind. Things independent of mind, the things-in-themselves, do exist. Percepts are caused by things in themselves and are ordered by our mental apparatus in space and time. Things-in-them-selves, which are the causes of sensations, are unknowable; they are not in space or time, nor are they substances.
In addition to the subjectivity of space and time, Kant believes in the subjectivity of these twelve categories divided into four sets of three: (1) of quantity. unity, plurality, totality, (2) of quality: reality, negation, limitation, (3) of relation: substance-and-accident, cause-and-effect, reciprocity, (4) of modality: possibility, existence, necessity (of these, al-Sadr mentions only causality). These, like time and space, are subjective. i.e. our mental constitution is such that they are applicable to whatever we experience, but there is no reason to suppose them applicable to things-in-themselves.
Mathematical propositions are all a priori. These are the only synthetic judgements which are a priori, because they rest not upon the variable and contingent content of experience but upon the unchanging forms of space and time in which all experience is given.
The statements of natural science, which are empirical and synthetic, are composed of two elements, one of which is empirical and the other rational. The empirical aspect relates to the content or stuff of experience, whereas the rational element relates to mind and its forms and categories. The natural sciences, according to Kant, do not describe the external order of things-in-themselves, but are valid and trustworthy within the realm of experience, i.e. the experienced order of 'things-in-us'.
Here al-Sadr does not appear to appreciate the depth of Kant's skepticism regarding the knowledge of the external world, which he interprets as a kind of relativism. Hence these statements of his: "Knowledge (in Kant's philosophy), therefore, is a mixture of subjectivity and objectivity", and "That is why relativity is imposed on every truth representing external things in our knowledge, in the sense that our knowledge indicates to us the thing's reality in us, and not the thing's reality in itself". He does not seem to notice that Kant's extreme subjectivism makes not only metaphysics impossible but so also natural science, which is reduced to some kind of phenomenology. Kant's subjectivism makes his realism altogether ineffectual. The things-in-themselves are shadows that lurk on the boundaries of his system, which is idealist and subjective through and through. His realism is as useless for science as his rationalism is useless for metaphysics and theology.
Al-Sadr's criticism, however, is addressed mainly to Kant's denial of the possibility of metaphysics. According to Kant, there can be no synthetic judgements relating to metaphysics. Empirical synthetic judgements, like that of the sciences, involve mind's formal modes and categories: space and time and the categories of quantity, quality, relation and modality. These finite categories apply to sense-experience and phenomena, not to things-in-themselves, the noumenal. God, soul, and the noumenal world lie beyond experience, and hence there can be no empirical synthetic judgements about them. Also, since the noumenal world transcends mind's a priori concepts, a priori synthetic judgements, like that of logic and mathematics, which are purely formal and empty of content, cannot pertain to metaphysics. Accordingly there is no room in metaphysics for anything but analytic judgements, which do not constitute any real knowledge at all.
Al-Sadr, it seems, does not notice that Kant has built the realm of the mind and experience into an almost autonomous and self-contained world by itself (almost, we said, allowing for Kant's inconsequential belief in the unknowable things-in-themselves, which cause sensations).
This is shown by the 'two basic errors' in Kant's theory that he points out. Firstly, he points out, Kant considers mathematical science to 'produce' mathematical truths and principles, which are above error and contradiction, whereas every realistic philosophy must recognize that science does not 'produce' or 'create' truths. Science is revelatory of what transcends the limits of mind. The propositions of mathematics reflect an objective reality and are, in this sense, similar to the laws of natural science. Secondly, "Kant considers the laws that have their foundation in the human mind as laws of the mind, and not scientific reflections of the objective laws that govern and regulate the world as a whole. They are nothing but relations present in the mind naturally, and used by the mind to organize its empirical knowledge." Such a position, al-Sadr says, leads to idealism, "for if the primary knowledge in the mind'.
Is nothing but dependent relations awaiting a subject in which to appear, then how could we move from conception to objectivity? Further, how could- we prove the objective reality of our various sense perceptions that is, the natural phenomena whose objectivity Kant admits?"
The fact is that Kant's position is already deeply steeped in idealism. In his system extreme rationalism leads to an inscrutable subjectivism. Kant, in the ultimate analysis, is not a relativist but a skeptic, if not altogether a sophist. Objectivity for him lies within the inner realm of experience. Knowledge, he would say in reply to al-Sadr, is indeed revelatory, but revelatory of that which is within this realm.