If man wishes to form to himself clear, perspicuous ideas of his soul, let him throw himself back on his experience--let him renounce his prejudices--let him avoid theological conjecture--let him tear the bandages which he has been taught to think necessary, but with which he has been blind-folded, only to confound his reason. If it be wished to draw man to virtue, let the natural philosopher, let the anatomist, let the physician, unite their experience; let them compare their observations, in order to show what ought to be thought of a substance, so disguised, so hidden by absurdities, as not easily to be known. Their discoveries may perhaps teach moralists the true motive-power that ought to influence the actions of man--legislators, the true motives that should actuate him, that should excite him to labour to the welfare of society--sovereigns, the means of rendering their subjects truly happy; of giving solidity to the power of the nations committed to their charge. Physical souls have physical wants, and demand physical happiness. These are real, are preferable objects, to that variety of fanciful chimeras, each in its turn giving place to the other, with which the mind of man has been fed during so many ages. Let us, then, labour to perfect the morality of man; let us make it agreeable to him; let us excite in him an ardent thirst for its purity: we shall presently see his morals become better, himself become happier; his soul become calm and serene; his will determined to virtue, by the natural, by the palpable motives held out to him. By the diligence, by the care which legislators shall bestow on natural philosophy, they will form citizens of sound understandings; robust and well constituted; who, finding themselves happy, will be themselves accessary to that useful impulse so necessary for their soul. When the body is suffering, when nations are unhappy, the soul cannot be in a proper state. _Mens sana in corpore sano_, a sound mind in a sound body, will be always able to make a good citizen. The more man reflects, the more he will be convinced that the soul, very far from being distinguished from the body, is only the body itself, considered relatively to some of its functions, or to some of the modes of existing or acting, of which it is susceptible whilst it enjoys life. Thus, the soul is man, considered relatively to the faculty he has of feeling, of thinking, of acting in a mode resulting from his peculiar nature; that is to say, from his properties, from his particular organization: from the modifications, whether durable or transitory, which the beings who act upon him cause his machine to undergo.