When we pass to the question of the postulates, you will certainly be surprised if I tell you that the disagreement between us consists in your wish to introduce a _metaphysical_ postulate into economic science; whereas I wish here to rule out every metaphysical postulate and to confine myself entirely to the analysis of the given facts. The accusation of being _metaphysical_ will seem to you the last that could ever be brought against you. Your implied metaphysical postulate is, however, this; that the facts of man's activity are of the same nature as physical facts; that in the one case as in the other we can only observe regularity and deduce consequences therefrom, without ever penetrating into the inner nature of the facts; that these facts are all alike _phenomena_ (meaning that they would presuppose a _noumena_, which evades us, and of which they are manifestations). Hence whereas I have called my essay 'On the economic _principle_,' yours is entitled 'On the economic _phenomenon_.' How could you defend this postulate of yours except by a _metaphysical_ monism; for example that of Spencer? But, whilst Spencer was anti-metaphysical and positivist in words, I claim the necessity of being so in deeds; and hence I cannot accept either _his_ metaphysics or _his_ monism, and I hold to experience. This testifies to me the fundamental distinction between external and internal, between physical and mental, between mechanics and teleology, between passivity and activity, and secondary distinctions involved in this fundamental one. What metaphysics unites philosophy distinguishes (and joins together); the abstract contemplation of unity is the death of philosophy. Let us confine ourselves to the distinction between physical and mental. Whilst the external facts of nature, admitted by empirical physical science, are always phenomena, since their source is by definition outside themselves, the internal facts or activities of man, cannot be called phenomena, since they are their own source.