I have on several occasions heard something like a feeling of distaste expressed for the endless discussions about value and the economic principle which absorb the energies of economic science. It is said that if this splitting of hairs over the scholastic accuracy of its principle were abandoned, the science might throw light on historical and practical questions which concern the welfare of human society. Apparently you have not allowed yourself to be alarmed by the threatened distaste of readers; nor indeed am I. Can we silence the doubts which disturb us? Could we have assurance whilst silencing these doubts that we were not endangering just those _practical_ issues which the majority have at heart? Issues which we ourselves have at heart since we are certainly not able, like the monks of old, to free ourselves from interest in _the affairs of the age_. May not science be, as Leibniz said, _quo magis speculativa, magis practica_? We must then go our way, and endeavour to satisfy our doubts, with all the caution and self-criticism of which we are capable; since they cannot be suppressed. On the other hand we should endeavour also not to offer our solutions to the public except when our knowledge,--wide if it may be so (yet necessarily imperfect)--of the literature on the subject, gives us some confidence that we are not repeating things already stated. Unless indeed, other considerations make us think it opportune to repeat and to impress things which have been stated, but without sufficient emphasis. The new school of economic thought, of which you are such a worthy representative, has a merit of no small significance. It has reacted against the anti-scientific tendencies of the historical and empirical schools, and has restored the concept of a science of _pure_ economics. This means indeed nothing more than a science which is science; the word pure, unless tautologous, is an explanation added for those who are ignorant or unmindful of what a science is. Economics is neither history nor discussion of practical issues: it is a science possessing its own principle, which is indeed called the _economic principle_. But, as I had occasion to remark at another time,[95] I do not consider that this principle whose fundamental character is asserted, has hitherto been grasped in its individuality, nor conveniently defined in relation to other groups of facts, that is to the principles of other sciences. Of those conceptions of it which seem to me _erroneous_, the chief ones can be reduced to four which I will call the _mechanical_, the _hedonistic_, the _technological_ and the _egoistic_. You have now rejected the first two, because you think that mechanical and hedonistic considerations belong to metaphysics and psychology. But I acknowledge that I am dissatisfied with your method of arriving at this praiseworthy rejection.