_Secondly_, We shall avow without the least hesitation, that men of the greatest genius, of the most indefatigable research, are not acquainted with the essence of stones, plants, animals, nor with the secret springs which constitute some, which make others vegetate or act: but then at least we either feel them or see them; our senses have a knowledge of them in some respects; we can perceive some of their effects; we have something whereby to judge of them, either accurately or inaccurately; we can conceive that which is matter, however varied, however subtle, however minute, by analogy with other matter; but our senses cannot compass that which is immaterial on any side; we cannot by any possible means understand it; we have no means whatever of ascertaining its existence; consequently we cannot even form an idea of it; such a being is to us an occult principle, or rather a being which imagination has composed, by deducting from it every known quality. If we are ignorant of the intimate combination of the most material beings, we at least discover, with the aid of experience, some of their relations with ourselves: we have a knowledge of their surface, their extent, their form, their colour, their softness, their density; by the impressions they make on our senses, we are capable of discriminating them--of comparing them--of judging of them in some manner--of seeing them--of either avoiding or courting them, according to the different modes in which we are affected by them; we cannot apply any of these tests to immaterial beings; to spirits; neither can those men who are unceasingly talking to mankind of these inconceivable things. _Thirdly_, We have a consciousness of certain modifications in ourselves, which we call sentiment, thought, will, passions: for want of being acquainted with our own peculiar essence; for want of precisely understanding the energy of our own particular organization, we attribute these effects to a concealed cause, distinguished from ourselves; which the theologians call a spiritual cause, inasmuch as it appears to act differently from our body. Nevertheless, reflection, experience, every thing by which we are enabled to form any kind of judgment, proves that material effects can only emanate from material causes. We see nothing in the universe but physical, material effects, these can only be produced by analogous causes; it is, then certainly more rational to attribute them to nature herself, of which we may know something, if we will but deign to meditate her with attention, rather than to spiritual causes, of which we must for ever remain ignorant, let us study them as long as we please.