David Hume Quote
There is an inconvenience which attends all abstruse reasoning. that it may silence, without convincing an antagonist, and requires the same intense study to make us sensible of its force, that was at first requisite for its invention. When we leave our closet, and engage in the common affairs of life, its conclusions seem to vanish, like the phantoms of the night on the appearance of the morning; and 'tis difficult for us to retain even that conviction, which we had attain'd with difficulty.
Part 1 Of virtue and vice in general, Sect. 1 Moral distinctions not deriv'd from reason - A Treatise of Human Nature (1739-40) - Of Morals