We should reject knowledge claims concerning matters of fact about the nature of the world which are not supportable by the evidence of experience. This leads to a tendency among empiricists to emphasize that the limit of human knowledge and imagination is bounded by the limit of our experience. Empiricists reject the rationalist claim that it is possible to come to know by a priori reason alone the nature of an intelligible real world inaccessible to experience that stands beyond appearances. The empiricist may argue that concepts (such as substance), and the terms that express them, are meaningless or else must relate to some possible experience, since concepts and terms get their meaning by reference to some possible experience, but a world beyond experience cannot be a world that might possibly be experienced; in either case it is not possible to use meaningful concepts to talk of a world beyond possible experiences.
The tendency in empiricism is also to deny the existence of natural necessity: necessity is a property only of logical relations between concepts, or of logical relations between ideas or thoughts, not between things or events in the world whose existence, nature and connections are all contingent; such natural contingent connections can be discovered not by reason, which can establish only necessary truths and necessary connections, but only by experience. Empiricism is inclined to argue that there are two exclusive and together exhaustive types of proposition.
(a) Propositions whose truth, logically speaking, can be known merely by understanding them, or by deductive reasoning alone, independently of the evidence of experience: truths of reason.
(b) Propositions whose truth, logically speaking, cannot be known merely by understanding them, or by deductive reasoning alone, but which depend on the evidence of experience: truths of fact.
All propositions which tell us anything about the real or actual world are truths of fact. Propositions stating matters of fact cannot be known to be true merely by our understanding them, or by our deducing them from other propositions known to be true by the understanding alone; if we can know them to be true at all, they must be known through consulting experience. It should be noted that the distinction is not the genetic one of how we come to have, acquire, or understand these different sorts of proposition, but a logical question concerning on what, once acquired or understood, the truth or falsity of a proposition depends, and on what knowledge of the truth or falsity of a proposition depends. If the truth or falsity of a proposition depends only on the meaning of the terms in it, then it is an a priori proposition whose truth or falsity can be known a priori by reason alone independently of empirical evidence. If the truth or falsity of a proposition does not depend only on the meaning of the terms in it, then it is an a posteriori proposition whose truth or falsity can only be known a posteriori by empirical evidence, not by reason alone. (Frederick Copleston, 1964, 54)
The basic contrast between rationalism and empiricism is an argument about the extent and nature of what truths it is logically possible to know a priori by the understanding independently of experience, by intellectual intuition