CHAPTER IV.  OF THE REALITY OF KNOWLEDGE.

1.  Objection.  ’Knowledge placed in our Ideas may be all unreal or chimerical’

I doubt not but my reader, by this time, may be apt to think that I have been all this while only building a castle in the air; and be ready to say to me:—­

’To what purpose all this stir?  Knowledge, say you, is only the perception of the agreement or disagreement of our own ideas:  but who knows what those ideas may be?  Is there anything so extravagant as the imaginations of men’s brains?  Where is the head that has no chimeras in it?  Or if there be a sober and a wise man, what difference will there be, by your rules, between his knowledge and that of the most extravagant fancy in the world?  They both have their ideas, and perceive their agreement and disagreement one with another.  If there be any difference between them, the advantage will be on the warm-headed man’s side, as having the more ideas, and the more lively.  And so, by your rules, he will be the more knowing.  If it be true, that all knowledge lies only in the perception of the agreement or disagreement of our own ideas, the visions of an enthusiast and the reasonings of a sober man will be equally certain.  It is no matter how things are:  so a man observe but the agreement of his own imaginations, and talk conformably, it is all truth, all certainty.  Such castles in the air will be as strongholds of truth, as the demonstrations of Euclid.  That an harpy is not a centaur is by this way as certain knowledge, and as much a truth, as that a square is not a circle.

’But of what use is all this fine knowledge of men’s own imaginations, to a man that inquires after the reality of things?  It matters not what men’s fancies are, it is the knowledge of things that is only to be prized:  it is this alone gives a value to our reasonings, and preference to one man’s knowledge over another’s, that it is of things as they really are, and not of dreams and fancies.’

2.  Answer Not so, where Ideas agree with Things.

To which I answer, That if our knowledge of our ideas terminate in them, and reach no further, where there is something further intended, our most serious thoughts will be of little more use than the reveries of a crazy brain; and the truths built thereon of no more weight than the discourses of a man who sees things clearly in a dream, and with great assurance utters them.  But I hope, before I have done, to make it evident, that this way of certainty, by the knowledge of our own ideas, goes a little further than bare imagination:  and I believe it will appear that all the certainty of general truths a man has lies in nothing else.

3.  But what shall be the criterion of this agreement?