Direct Realism as a theory of perception has traditionally been thought to collapse on the existence of hallucinations. The cause of that collapse is what is familiar to philosophers as the Argument from Illusion. And what sustains that argument is the equally familiar No-Intrinsic-Difference Claim. The argument and the claim conspire to undermine Direct Realism as follows. We are first given cases in which we are acquainted with perceptual states of affairs that can be neither material bodies nor parts of such bodies. We are told that what we see in such cases is phenomenally indistinguishable from what we see in cases of veridical perception: There is, so the argument goes, no characteristic of either kind of state of affairs available to inspection which enables us to distinguish one from the other. And since there is no phenomenal difference between them, there is also no intrinsic difference; hence, we are never directly aware of material bodies or their parts in perception—from which it is generally inferred that Direct Realism is false.

Professor Copi provides us with the following example:

In successive issues of this journal Ronald Munson [2] and I [4] have made, quite independently, conflicting claims about the relationship between biological adaptation and biological function. I state, admittedly without proof, that “a functional statement in biology draws attention to the fact that what is under consideration is an adaptation or something which confers an ‘adaptive advantage’ on its possessor” ([4], p. 89). This was an identity claim. Munson claims, with proof, that “adaptation and function are not identical” ([2], p. 205). In this discussion note I want to examine the three arguments Munson gives in support of his claim, and I shall try to show that they do not hold.

Stegmüller's most recent publication, Theorie und Erfahrung, is the second of four volumes of a work which appears under the general title Probleme und Resultate der Wissenschaftstheorie und analytische Philosophie, ([36], all references in the discussion below are taken from the second volume of this work). In this voluminous and daring work the author intends to deliver a systematic, critical account of the most important literature which has appeared on various basic topics of philosophy of science and its underlying assumptions over the past twenty-five years. The work promises to become a classic in the German language because of its comprehensiveness, thoroughness, and clarity.

Michael Ruse [3] has criticized the distinction between biological function and evolutionary adaptation that I argued for in my article “Biological Adaptation” [2]. I shall show below that Ruse's criticisms are not, for the most part, well taken and that the distinction remains as I made it.

The problem of idealization in empirical sciences is very rarely taken up in works concerned with the methodology of those sciences. It seems to be common knowledge that in advanced natural sciences references are made to concepts such as “perfectly rigid body,” “material point,” “perfect gas,” etc., but it remains a fact that the most important methodological concepts, concepts which have determined the present-day form of the philosophy of science, have been advanced without regard to the peculiarities of the procedure mentioned above. And yet a moment's consideration suffices to raise doubts as to Carnap's physicalism and/or Popper's falsificationism if we just realize the trivial and commonly known truth that empirical sciences resort to idealizationism. For how are we to imagine a reduction of idealizational concepts to observation terms? And how are we to imagine that theorems which include idealizational concepts imply observation statements which refute, or at least are at variance with, those theorems? But it is a fact that such doubts have neither been raised by the authors of the conceptions mentioned above, nor have they been raised (as far as the present writer is informed) against them.