you pick out one or another of the series of sounds, an act which is indeed made possible by the natural analytic tendency of the human auditory sense, but which does not take place without a noticeable effort of attention. When you are learning a foreign language, and are for a while much among those who speak it, there comes a time when your ear and mind are well enough trained to follow and understand ordinary speakers with only a little effort of attention; but yet, at this stage, you are able, by simply withdrawing your attention a mere trifle, to let very common phrases run through your sense without your understanding them one whit. You can thus, by a slight change of attention, convert the foreign language from a jargon into a familiar speech, and back again into a jargon, just as, in the fixed visual field, you can make yourself see an object pretty plainly, or lose it altogether, by ceasing to give attention.
All these instances, which could be indefinitely multiplied, prove, first, that what we call attention modifies the knowledge that we at any moment get; and secondly, that this modification, through attention, may take place without any change in the impressions that at any moment come from without. The first stage in getting knowledge from bare sense-impressions is therefore the modification of sense by attention — a process belonging wholly to the subjective side; i.e. to our own minds.
But what is attention? and how does it modify sensation? Apparently, attention in the previous instances has been merely a power to increase or to
