That man should be uninterruptedly happy, it would be requisite that his powers were infinite; it would require that to his mobility he joined a vigor, attached a solidity, which nothing could change; or else it is necessary that the objects from which he receives impulse, should either acquire or lose properties, according to the different states through which his machine is successively obliged to pass; it would need that the essences of beings should be changed in the same proportion as his dispositions; should be submitted to the continual influence of a thousand causes, which modify him without his knowledge, and in despite of himself. If, at each moment, his machine undergoes changes more or less marked, which are ascribable to the different degrees of elasticity, of density, of serenity of the atmosphere; to the portion of igneous fluid circulating through his blood; to the harmony of his organs; to the order that exists between the various parts of his body; if, at every period of his existence, his nerves have not the same tensions, his fibres the same elasticity, his mind the same activity, his imagination the same ardour, &c. it is evident that the same causes in preserving to him only the same qualities, cannot always affect him in the same manner. Here is the reason why those objects that please him in one season displease him in another: these objects have not themselves sensibly changed; but his organs, his dispositions, his ideas, his mode of seeing, his manner of feeling, have changed:--such is the source of man's inconstancy.
If the same objects are not constantly in that state competent to form the happiness of the same individual, it is easy to perceive that they are yet less in a capacity to please all men; or that the same happiness cannot be suitable to all. Beings already various by their temperament, unlike in their faculties, diversified in their organization, different in their imagination, dissimilar in their ideas, of distinct opinions, of contrary habits, which an infinity of circumstances, whether physical or moral, have variously modified, must necessarily form very different notions of happiness. Those of a MISER cannot be the same as those of a PRODIGAL; those of a VOLUPTUARY, the same as those of one who is PHLEGMATIC; those of an intemperate, the same as those of a rational man, who husbands his health. The happiness of each, is in consequence composed of his natural organization, and of those circumstances, of those habits, of those ideas, whether true or false, that have modified him: this organization and these circumstances, never being the same in any two men, it follows, that what is the object of one man's views, must be indifferent, or even displeasing to another; thus, as we have before said, no one can be capable of judging of that which may contribute to the felicity of his fellow man.