From Hume’s Essays, Moral, Political, and Literary, Part II, Essay II, Of Refinement in the Arts:
Luxury is a word of an uncertain signification, and may be taken in a good as well as in a bad sense. In general, it means great refinement in the gratification of the senses; and any degree of it may be innocent or blameable, according to the age, or country, or condition of the person. The bounds between the virtue and the vice cannot here be exactly fixed, more than in other moral subjects. [...]
Since luxury may be considered either as innocent or blameable, one may be surprized at those preposterous opinions, which have been entertained concerning it; while men of libertine principles bestow praises even on vicious luxury, and represent it as highly advantageous to society; and on the other hand, men of severe morals blame even the most innocent luxury, and represent it as the source of all the corruptions, disorders, and factions, incident to civil government. We shall here endeavour to correct both these extremes, by proving, first, that the ages of refinement are both the happiest and most virtuous; secondly, that wherever luxury ceases to be innocent, it also ceases to be beneficial; and when carried a degree too far, is a quality pernicious, though perhaps not the most pernicious, to political society. [...]
The encrease and consumption of all the commodities, which serve to the ornament and pleasure of life, are advantageous to society; because, at the same time that they multiply those innocent gratifications to individuals, they are a kind of storehouse of labour, which, in the exigencies of state, may be turned to the public service. In a nation, where there is no demand for such superfluities, men sink into indolence, lose all enjoyment of life, and are useless to the public, which cannot maintain or support its fleets and armies, from the industry of such slothful members. [...]
Luxury, when excessive, is the source of many ills; but is in general preferable to sloth and idleness, which would commonly succeed in its place, and are more hurtful both to private persons and to the public. When sloth reigns, a mean uncultivated way of life prevails amongst individuals, without society, without enjoyment.
Rousseau : « ou le luxe est l’effet des richesses, ou il les rend nécessaires ; il corrompt à la fois le riche et le pauvre, l’un par la possession, l’autre par la convoitise ».
Voltaire : « Si l’on entend par luxe tout ce qui est au-delà du nécessaire, le luxe est une suite naturelle des progrès de l’espèce humaine ».