freedom from uncertainty what undefined simple notions and undemonstrated postulates we have to employ in the science and to express these ultimates without ambiguity. 'Evolutionary science,' rich as is its information about the details of the processes going on in the organic world, seems still to await its Frege or Russell. It talks, for example, much of 'hereditary' and non-hereditary peculiarities, and some of us can remember a time when our friends among the biologists seemed almost ready to put each other to the sword for differences of opinion about the inheritability of certain characteristics; but no one seems to trouble himself much with the question a philosopher would think most important of all--precisely what is meant by the metaphor of 'inheritance' when it is applied to the facts of biology. (Indeed, it is still quite fashionable to talk not merely as if a 'character' were, like a house or an orchard, a _thing_ which can be transferred bodily from the possession of a parent to the possession of the offspring, but even as though an 'heir' could 'inherit' himself.) This last remark leads me to a further consideration. Science and Philosophy are alike created by the simple determination to be _thorough_ in our thinking about the problems which all things and events present to us, to use no terms whose meaning is ambiguous, to assert no propositions as true until we are satisfied that they are either directly apprehended as true, or strictly deducible from other propositions which are thus apprehended. But now that the area of facts open to our exploration has become far too vast for a modern Francis Bacon to 'take all knowledge for his province', and convenience has led to the distinction between the philosopher and the man of science, a _practical_ distinction between the two makes its appearance. It is _convenient_ that our knowledge of detail should be steadily extended by considering the consequences which follow from a given set of postulates without waiting for the solution of the more strictly philosophical questions whether our postulates have been reduced to the simplest and most unambiguous expression, whether the list might not be curtailed by showing that some of its members which have been accepted on their own merits can be deduced from the rest, or again enlarged by the express addition of principles which we have all along been using without any actual formulation of them. The point may be illustrated by considering the set of 'postulates' explicitly made in the geometry of Euclid. We cannot be said to have made geometry thoroughly scientific until we know whether the traditional list of postulates is complete, whether some of the traditional postulates might not be capable of demonstration, and whether geometry as a science would be destroyed by the denial of one or more of the postulates. But it would be very undesirable to suspend examination of the consequences which follow from the Euclidean