t_: and when Beattie, Oswald, Reid,
&c. were exhausting themselves in proofs of the indispensableness of
this idea, they were fighting with shadows; for no man had ever
questioned the practical necessity for such an idea to the coherency
of human thinking. Not the practical necessity, but the internal
consistency of this notion, and the original right to such a notion,
was the point of inquisition. For, attend, courteous reader, and three
separate propositions will set before your eyes the difficulty. _First
Prop._, which, for the sake of greater precision, permit me to throw
into Latin:--_Non datur aliquid_ [A] _quo posito ponitur aliud_ [B] _a
priori_; that is, in other words, You cannot lay your hands upon that
one object or phenomenon [A] in the whole circle of natural
existences, which, being assumed, will entitle you to assume _a
priori_, any other object whatsoever [B] as succeeding it. 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 accompanime