&c._

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