You could not, I say, of any object or phenomenon whatever, assume this succession _a priori_--that is, _previously to experience_. _Second Prop._ But, if the succession of B to A be made known to you, not _a priori_ (by the involution of B in the idea of A), but by experience, then you cannot ascribe _necessity_ to the succession: the connection between them is not necessary but contingent. For the very widest experience--an experience which should stretch over all ages, from the beginning to the end of time--can never establish a _nexus_ having the least approximation to necessity; no more than a rope of sand could gain the cohesion of adamant, by repeating its links through a billion of successions. _Prop. Third._ Hence (_i. e._ from the two preceding propositions), it appears that no instance or case of _nexus_ that ever can have been offered to the notice of any human understanding, has in it, or, by possibility, could have had anything of necessity. Had the _nexus_ been necessary, you would have seen it beforehand; whereas, by Prop. I. _Non datur aliquid, quo posito ponitur aliud a priori._ This being so, now comes the startling fact, that the notion of a _cause_ includes the notion of necessity. For, if A (the cause) be connected with B (the effect) only in a casual or accidental way, you do not feel warranted in calling it a cause. If heat applied to ice (A) were sometimes followed by a tendency to liquefaction (B) and sometimes not, you would not consider A connected with B as a cause, but only as some variable accompaniment of the true and unknown cause, which might allowably be present or be absent. This, then, is the startling and mysterious phenomenon of the human understanding--that, in a certain notion, which is indispensable to the coherency of our whole experience, indispensable to the establishing any _nexus_ between the different parts and successions of our whole train of notices, we include an accessary notion of necessity, which yet has no justification or warrant, no assignable derivation from any known or possible case of human experience. We have one idea at least--viz. the idea of causation--which transcends our possible experience by one important element, the element of _necessity_, that never can have been derived from the only source of ideas recognised by the philosophy of this day. A Lockian never can find his way out of this dilemma. The experience (whether it be the experience of sensation or the experience of reflection) which he adopts for his master-key, never will unlock this case; for the sum total of human experience, collected from all ages, can avail only to tell us what _is_, but never what _must be_. The idea of necessity is absolutely transcendant to experience, _per se_, and must be derived from some other source. From what source? Could Hume tell us? No: he, who had started the game so acutely (for with every allowance for the detection made in Thomas Aquinas, of the original suggestion, as