That we cannot know there is a God is a conclusion to which we have seen the decline of rationalism and the growth of empiricism slowly tending. But that we ought to live as though there were a God--what can such a phrase mean? What manner of life does it prescribe? Above all, what sort of an _ought_ is this and how does it bind us? There is no deeper interest for Kant than that which invites one to consider the meaning of the word “ought.” I say, _the_ meaning of “ought,” yet it may be that the word has more than one meaning. For compare these two examples of its use,--first this: If you want to bisect a line you ought to describe certain arcs and draw a certain straight line. And then this: “You ought to speak the truth.” We notice at once a rhetorical difference in these two uses of the _ought_. In the first, a certain procedure is commanded _if_ and only _if_ we want to bisect a line. Leave out the condition this _if_ introduces, and the _ought_ with all that follows on it loses its meaning. No decalogue could be imagined to contain among its commands an injunction to describe arcs and draw lines. Let us call this use of the _ought_ the hypothetical use, let us call the command such an _ought_ introduces a hypothetical command or in Kant’s own phrase a “_hypothetical imperative_.” An _ought_ that is inseparable from an _if_ is a hypothetical imperative. On the other hand when I say, “You ought to speak the truth,” “You ought not to steal,” I seem to be using the _ought_ in a sense that needs no _if_ to make its meaning clear. More than that, attempts to supply an _if_, so far from making the meaning of the _ought_ clearer, have more often than not the effect of changing, of travestying the meaning we instinctively see in it. Truthful speaking and honest dealing be indeed useful devices for getting along in the world, but one who is honest because honesty is the best policy seems to us hardly honest--at all events he seems to have missed the point that honesty is enjoined on us without _ifs_ or _buts_. The obligation to be honest is an unconditional command, a “_categorical imperative_.” It is of such stuff as decalogues are made on--it is so the voice of duty speaks in us. It needs no pointing out that so far as our examples go, the hypothetical ought has no moral flavor. No sin attaches to one who has left undone the things he ought to have done _if_ he aimed at bisecting a line. Sin does attach to one who has done what he ought not to have done in the way of lying, no matter what end seemed to justify the means. This hypothetical _ought_ finds its reason in pure science, this categorical in pure morality.