And from this recognition is inferred in its turn the true criticism of the _hedonistic_ conception of the economic principle. You say that 'the equations of pure economics express merely the fact of a choice, and can be drawn up independently of the ideas of pleasure and pain,' but you admit at the same time that the fact of the _choice_ 'can be expressed equally well as a fact of _pleasure_.' It is true that every case of economic choice is at the same time, a case of _feeling_: of agreeable feeling if the economic choice is rightly made, of disagreeable feeling, if it is ill made. Man's activity develops itself in the human mind, not under a pneumatic bell, and an activity which develops rightly, brings as its reflex, a feeling of pleasure, that which develops badly, one of displeasure. What is economically useful, is, at the same time _pleasurable_. But this judgment cannot be converted. The pleasurable is not always economically useful. The mistake in the hedonist theory consists in making this conversion. Pleasure may appear unaccompanied by man's activity, or may be accompanied by a human activity which is not economic. Herein lies the fundamental distinction between _pleasure_ and _choice_. A choice, is in the concrete, inseparable from the feeling of pleasure and displeasure; but this feeling is separable from _choice_, and may in fact exist independently of it. If psychology be understood (as it is usually understood) as the science of psychical mechanism, economics is not a psychological science; this Herr von Ehrenfels fails to grasp. I do not know whether you have read the two volumes hitherto published on the _System der Werttheorie_.[96] After devoting some hundred pages to psychological disquisitions--which I do not mean to discuss here--he wishes, finally, to prove that his definitions of value remain sound, from whatever theory of psychology you start. He does this as he asserts (§ 87), not because he is doubtful of himself, but to safeguard his economic conclusions, which are so important for the practical problems of life, from unjustified attacks based on the standpoints of schools of psychology other than his own, the method of the barrister, who composes an apparent conclusion, and makes several demands that are connected therewith _subordinately_. It is true that there is no need for economists to spend their time on details of theoretical psychology; so true that Professor Ehrenfels might spare us his: but is it not true that economics remains the same _whatever psychological theory is accepted_. The unity of science means that a modification at one point is never without some reaction on the others; and the reaction is greatest when it is a question of the way of conceiving two facts, distinct but inseparable, like the economic and the psychical fact.