"Nothing is more difficult and requires more care than philosophical deduction, nor is there anything more adverse to its accuracy than fixidity of opinion. The man who is certain he is right is almost sure to be wrong, and he has the additional misfortune of inevitably remaining so. All our theories are fixed upon uncertain data, and all of them want alteration and support. Ever since the world began opinion has changed with the progress of things; and it is something more than absurd to suppose that we have a sure claim to perfection, or that we are in possession of the highest stretch of intellect which has or can result from human thought. Why our successors should not displace us in our opinions, as well as in our persons, it is difficult to say; it ever has been so, and from analogy would be supposed to continue so; and yet, with all this practical evidence of the fallibility of our opinions, all, and none more than philosophers, are ready to assert the real truth of their opinions." In his discourse entitled "Observations on Mental Education," like a skilful physician he first determines what is the great intellectual disease from which the community suffers--"deficiency of judgment,"--and then he lays down rules by which each man may attempt his own cure. For this self-education, "it is necessary that a man examine himself, and that not carelessly.... A first result of this habit of mind will be an internal conviction of _ignorance in many things respecting which his neighbours are taught_, and that his opinions and conclusions on such matters ought to be advanced with reservation. A mind so disciplined will be _open to correction upon good grounds in all things_, even in those it is best acquainted with; and should familiarize itself with the idea of such being the case.... It is right that we should stand by and act on our principles, but not right to hold them in obstinate blindness, or retain them when proved to be erroneous." And then he gives cases from his own mental history:--"I remember the time when I believed a spark was produced between voltaic metals as they approached to contact (and the reasons why it might be possible yet remain); but others doubted the fact and denied the proofs, and on re-examination I found reason to admit their corrections were well founded. Years ago I believed that electrolites could conduct electricity by a conduction proper; that has also been denied by many through long time: though I believed myself right, yet circumstances have induced me to pay that respect to criticism as to re-investigate the subject, and I have the pleasure of thinking that nature confirms my original conclusions. So, though evidence may appear to preponderate extremely in favour of a certain decision, it is wise and proper to hear a counter-statement. You can have no idea how often, and how much, under such an impression, I have desired that the marvellous descriptions which have reached me might prove, in some