On the same day as Hume wrote this letter from Edinburgh, Gibbon wrote from London to Adam Ferguson and said among other things, "What an excellent work is that with which our common friend Mr. Adam Smith has enriched the public! An extensive science in a single book, and the most profound ideas expressed in the most perspicuous language. He proposes visiting you very soon, and I find he means to exert his most strenuous endeavours to persuade Mr. Hume to return with him to town. I am sorry to hear that the health and spirits of that truly great man are in a less favourable state than his friends could wish, and I am sure you will join your efforts in convincing him of the benefits of exercise, dissipation, and change of air." Some of Smith's personal friends seem to have entertained the common prejudice that a good work on commerce could not be reasonably expected from a man who had never been engaged in any branch of practical business, and seemed in outward air and appearance so ill fitted to succeed in such a line of business if he had engaged in it. One of these was Sir John Pringle, President of the Royal Society, and formerly, like Smith himself, Professor of Moral Philosophy at a Scotch university. When the _Wealth of Nations_ appeared Sir John Pringle remarked to Boswell that Smith, having never been in trade, could not be expected to write well on that subject any more than a lawyer upon physic, and Boswell repeated the remark to Johnson, who at once, however, sent it to the winds. "He is mistaken, sir," said the Doctor; "a man who has never been engaged in trade himself may undoubtedly write well upon trade, and there is nothing that requires more to be illustrated by philosophy than does trade. As to mere wealth--that is to say, money--it is clear that one nation or one individual cannot increase its store but by making another poorer; but trade procures what is more valuable, the reciprocation of the peculiar advantages of different countries. A merchant seldom thinks but of his own particular trade. To write a good book upon it a man must have extensive views; it is not necessary to have practised to write well upon a subject."