(_c_) But then our troubles are not ended. Only the essential and invariable conditions enter into the cause, if the {165} cause is invariable. This begins to cut away the particular circumstances of the case. You need not use the trigger, nor even the cap; you may ignite powder in many ways. You may have many kinds of explosives. All that is essential is to have an explosion of a certain force and not too great rapidity. Then you will get this paradox. What is merely essential to the effect, is always something less than any combination of real “things” which will produce the effect, because every real thing has many properties irrelevant to this particular effect. So, _if the cause means something real_, as a material object is real, it cannot be invariable and essential. If it is not something real, and is essential, it fines down into a reason or law--the antecedent in a hypothetical judgment. _Ground, or real system with known laws_ (_d_) We can only escape this by identifying both cause and reason with the complete ground; that is, the nature of a system of reality within which the cause and effect both lie. But even then, though the ground is _real_, it is not antecedent in time. We see, indeed, that the conditions of an effect must be continuous through the effect. If the process were taken as cut in two at any point, its connection would be destroyed. If _a_ cause and _b_ effect were really detached events, what difference could it make if, instead of _a_, _c_ preceded _b_? _Postulate of Knowledge_ 4. The postulate of Knowledge, then, is very badly stated as Uniformity of Nature. That was due to the vulgar notion of Inductive “generalisation.” It must be stated in two parts: first, “Once true always true;” and secondly, “Our truth is enough for us,” that is, it covers enough of the universe for our practical and theoretical needs. The {166} two parts may be put together by saying, “The universe is a rational system,” taking rational to mean not only of such a nature that it can be known by intelligence, but further of such a nature that it can be known and handled by our intelligence. _Conclusion_ 5. These lectures have been unavoidably descriptive rather than thorough, and yet, as I warned you, descriptive of properties which are in a sense not at all new, but quite familiar, and even trite. You will not feel, at first, that the full interest which I claimed for the science of knowledge, really attaches to these dry relations of abstract thought. You will get no permanent good unless you carry the study forward for yourselves, and use these ideas as a clue to find your bearings in the great world of knowledge.