I am using the word "perceive". I am using it here in such a way that to say of an object that it is perceived does not entail saying that it exists in any sense at all. And this is a perfectly correct and familiar usage of the word. If there is thought to be a difficulty here, it is perhaps because there is also a correct and familiar usage of the word "perceive", in which to say of an object that it is perceived does carry the implication that it exists.
That all our knowledge begins with experience there can be no doubt. For how is it possible that the faculty of cognition should be awakened into exercise otherwise than by means of objects which affect our senses, and partly of themselves produce representations, partly rouse our powers of understanding into activity, to compare, to connect, or to separate these, and so to convert the raw material of our sensuous impressions into a knowledge of objects, which is called experience? In respect of time, therefore, no knowledge of ours is antecedent to experience, but begins with it. But though all our knowledge begins with experience, it by no means follows, that all arises out of experience. For, on the contrary, it is quite possible that our empirical knowledge is a compound of that which we receive through impressions, and that which the faculty of cognition supplies from itself (sensuous impressions giving merely the occasion)... It is, therefore, a question which requires close investigation, and is not to be answered at first sight,—whether there exists a knowledge altogether independent of experience, and even of all sensuous impressions? Knowledge of this kind is called à priori, in contradistinction to empirical knowledge which has its sources à posteriori, that is, in experience.
Immanuel Kant, Critique of Pure Reason (1781, J.M.D. Meiklejohn Tr. 1872) Introduction I. Of the Difference Between Pure and Empirical Knowledge.
Once the increase of empirical knowledge, and more exact modes of thought, made sharper divisions between the sciences inevitable, and once the increasingly complex machinery of the state necessitated a more rigorous separation of ranks and occupations, then the inner unity of human nature was severed too.
The philosophy of Parmenides is a strange blend of mysticism and logic. It is mysticism, for its goal is not the gradual and cumulative correction of empirical knowledge, but deliverance from it through the instantaneous and absolute grasp of "immovable" truth. This is not the way of techne, but the way of revelation: it lies "beyond the path of men" (B. 1.27). Yet this revelation is itself addressed to man's reason and must be judged by reason. Its core is pure logic: a rigorous venture in deductive thinking, the first of its kind in European thought. This kind of thinking could be used against the world of the senses … This projection of the logic of Being upon the alien world of Becoming was Parmenides' most important single contribution to the history of thought, though it is seldom recognized as such. Without it, his doctrine of Being could have remained a speculative curiosity. With it, he laid the foundations for the greatest achievement of the scientific imagination of Greece, the atomic hypothesis.
The capacity to distinguish between empirical knowledge and value-judgments, and the fulfillment of the scientific duty to see the factual truth as well as the practical duty to stand up for our own ideals constitute the program to which we wish to adhere with ever increasing firmness.
Max Weber, Objectivity in Social Science and Social Policy (1904)