If indeed every phenomenon was generally the effect of no more than one cause, a knowledge of the law of that cause would, unless there was a logical error in our reasoning, enable us confidently to predict all the circumstances of the phenomenon. We might then, if we had carefully examined our premises and our reasoning, and found no flaw, venture to disbelieve the testimony which might be brought to show that matters had turned out differently from what we should have predicted. If the causes of erroneous conclusions were always patent on the face of the reasonings which lead to them, the human understanding would be a far more trustworthy instrument than it is. But the narrowest examination of the process itself will help us little towards discovering that we have omitted part of the premises which we ought to have taken into our reasoning. Effects are commonly determined by a _concurrence_ of causes. If we have overlooked any one cause, we may reason justly from all the others, and only be the further wrong. Our premises will be true, and our reasoning correct, and yet the result of no value in the particular case. There is, therefore, almost always room for a modest doubt as to our practical conclusions. Against false premises and unsound reasoning, a good mental discipline may effectually secure us; but against the danger of _overlooking_ something, neither strength of understanding nor intellectual cultivation can be more than a very imperfect protection. A person may be warranted in feeling confident, that whatever he has carefully contemplated with his mind's eye he has seen correctly; but no one can be sure that there is not something in existence which he has not seen at all. He can do no more than satisfy himself that he has seen all that is visible to any other persons who have concerned themselves with the subject. For this purpose he must endeavour to place himself at their point of view, and strive earnestly to see the object as they see it; nor give up the attempt until he has either added the appearance which is floating before them to his own stock of realities, or made out clearly that it is an optical deception. * * * * *