The Wealth of Nations by Part 5 Chapter 2 Page 155

the emoluments of inferior trades. A tax upon this recompense, therefore, could have no other effect than to raise it somewhat higher than in proportion to the tax.

If it did not rise in this manner, the ingenious arts and the liberal professions, being no longer upon a level with other trades, would be so much deserted that they would soon return to that level.

The emoluments of offices are not, like those of trades and professions, regulated by the free competition of the market, and do not, therefore, always bear a just proportion to what the nature of the employment requires. They are, perhaps, in most countries, higher than it requires; the persons who have the administration of government being generally disposed to reward both themselves and their immediate dependants rather more