Degrees of empiricism
Empiricism, whether concerned with concepts or knowledge, can be held with varying degrees of strength. On this basis, absolute, substantive, and partial empiricisms can be distinguished.
Absolute empiricism
Absolute empiricists hold that there are no a priori concepts, either formal or categorial, and no a priori beliefs or propositions. Absolute empiricism about the former is more common than that about the latter, however. Although nearly all Western philosophers admit that obvious tautologies (e.g., “all red things are red”) and definitional truisms (e.g., “all triangles have three sides”) are a priori, many of them would add that these represent a degenerate case.
Substantive empiricism
A more moderate form of empiricism is that of the substantive empiricists, who are unconvinced by attempts that have been made to interpret formal concepts empirically and who therefore concede that formal concepts are a priori, though they deny that status to categorial concepts and to the theoretical concepts of physics, which they hold are a posteriori. According to this view, allegedly a priori categorial and theoretical concepts are either defective, reducible to empirical concepts, or merely useful “fictions” for the prediction and organization of experience.
The parallel point of view about knowledge assumes that the truth of logical and mathematical propositions is determined, as is that of definitional truisms, by the relationships between meanings that are established prior to experience. The truth often espoused by ethicists, for example, that one is truly obliged to rescue a person from drowning only if it is possible to do so, is a matter of meanings and not of facts about the world. On this view, all propositions that, in contrast to the foregoing example, are in any way substantially informative about the world are a posteriori. Even if there are a priori propositions, they are formal or verbal or conceptual in nature, and their necessary truth derives simply from the meanings that attached to the words they contain. A priori knowledge is useful because it makes explicit the hidden implications of substantive, factual assertions. But a priori propositions do not themselves express genuinely new knowledge about the world; they are factually empty. Thus “All bachelors are unmarried” merely gives explicit recognition to the commitment to describe as unmarried anyone who has been described as a bachelor.
Substantive empiricism about knowledge regards all a priori propositions as being more-or-less concealed tautologies. If a person’s “duty” is thus defined as that which he should always do, the statement “A person should always do his duty” then becomes “A person should always do what he should always do.” Deductive reasoning is conceived accordingly as a way of bringing this concealed tautological status to light. That such extrication is nearly always required means that a priori knowledge is far from trivial.
For the substantive empiricist, truisms and the propositions of logic and mathematics exhaust the domain of the a priori. Science, on the other hand—from the fundamental assumptions about the structure of the universe to the singular items of evidence used to confirm its theories—is regarded as a posteriori throughout. The propositions of ethics and those of metaphysics, which deals with the ultimate nature and constitution of reality (e.g., “only that which is not subject to change is real”), are either disguised tautologies or “pseudo-propositions”—i.e., combinations of words that, despite their grammatical respectability, cannot be taken as true or false assertions at all.
Partial empiricism
The least thoroughgoing type of empiricism here distinguished, ranking third in degree, can be termed partial empiricism. According to this view, the realm of the a priori includes some concepts that are not formal and some propositions that are substantially informative about the world. The theses of the transcendental idealism of Immanuel Kant (1720–1804), the general scientific conservation laws, the basic principles of morality and theology, and the causal laws of nature have all been held by partial empiricists to be both “synthetic” (substantially informative) and a priori. As noted above, philosophers who embrace the Kripkean notion of reference fixing would add to this class propositions such as “heat is the cause of sensations of warmth” and “Aristotle was the teacher of Alexander the Great,” both of which derive their presumed aprioricity from the hypothetical circumstances in which their subject terms were introduced. At any rate, in all versions of partial empiricism there remain a great many straightforwardly a posteriori concepts and propositions: ordinary singular propositions about matters of fact and the concepts that figure in them are held to fall in this domain.