s of
thoughts goes on in our minds--is the general phenomenon to be
considered. Without, as yet, pronouncing that sensations and copies of
sensations will turn out to form the whole contents of our
consciousness, he tries to show for what part of those contents they
will account.

Here we come to the doctrine which for him and his school gave the
key to all psychological problems. It was James Mill's real merit,
according to his son, that he carried the principle of association of
ideas further than it had been carried by Hartley or other
predecessors.[514] The importance of the doctrine, indeed, is implied
in the very statement of the problem. If it be true, or so far as it
is true, that our consciousness reveals to us simply a series of
'sensations' and 'ideas,' the question must be how they are combined.
'Thought succeeds thought, idea follows idea incessantly,'[515] says
Mill; and this phrase assumes 'thoughts' and 'ideas' to be separable
atoms. How, then, do they come to coalesce into an apparently
continuous stream? The mind is a stream of 'ideas.' If the stream is
composed of drops, we must, of course, consider the drops as composing
the stream. The question is, What laws can we assign which will
determine the process of composition? The phrase 'association'
admittedly expresses some general and very familiar truths.
Innumerable connections may be established when there is no assignable
ground of connection in the ideas themselves other than the fact of a
previous contact. One idea not only calls up the other, but in some
way generates a belief in an independent connection. We hear thunder,
for example, and think of lightning. The two ideas are entirely
distinct and separate, for they are due to different senses. Yet we
not only think of lightning when we hear thunder, but we have no doubt
that there is a causal connection. We believe in this connection,
again, though no further explanation can be given of the fact.
Thunder and lightning have occurred together, and we infer that they
will, and even must, occur together. When we examine our whole
structure of belief, we find such 'arbitrary' associations pervade it
in every direction. Language itself is learned simply by association.
There is no connection whatever between the sound of the word 'man'
and the 'ideas' which the word excites, beyond the fact that the sound
has been previously heard when the ideas were excited. Here, then, is
a phenomenon to be expl