By the mere light of reason it seems difficult to prove the Immortality of the Soul. The arguments for it are commonly derived either from metaphysical topics, or moral or physical. But in reality, it is the gospel, and the gospel alone, that has brought life and immortality to light.
I. Metaphysical topics are founded on the supposition that the soul is immaterial, and that it is impossible for thought to belong to a material substance.
But just metaphysics teach us, that the notion of substance is wholly confused and imperfect, and that we have no other idea of any substance than as an aggregate of particular qualities, inhering in an unknown something. Matter, therefore, and spirit are at bottom equally unknown; and we cannot determine what qualities may inhere in the one or in the other.They likewise teach us, that nothing can be decided a priori concerning any cause or effect; and that experience being the only source of our judgments of this nature, we cannot know from any other principle, whether matter, by its structure or arrangement, may not be the cause of thought. Abstract reasonings cannot decide any question of fact or existence.2 But admitting a spiritual substance to be dispersed throughout the universe, like the etherial fire of the Stoics, and to be the only inherent subject of thought; we have reason to conclude from analogy, that nature uses it after the same manner she does the other substance, matter. She employs it as a kind of paste or clay; modifies it into a variety of forms and existences; dissolves after a time each modification; and from it’s substance erects a new form. As the same material substance may successively compose the body of all animals, the same spiritual substance may compose their minds: Their consciousness, or that system of thought, which they formed  during life, may be continually dissolved by death; and nothing interest them in the new modification. The most positive asserters of the mortality of the soul, never denied the immortality of its substance. And that an immaterial substance, as well as a material, may lose its memory or consciousness appears, in part, from experience, if the soul be immaterial. Reasoning from the common course of nature, and without supposing any new interposition of the supreme cause, which ought always to be excluded from philosophy; what is incorruptible must also be ingenerable. The soul, therefore, if immortal, existed before our birth: And if the former state of existence no wise concerned us, neither will the latter. Animals undoubtedly feel, think, love, hate, will, and even reason, tho’ in a more imperfect manner than man. Are their souls also immaterial and immortal? II. Let us now consider the moral arguments, chiefly those arguments derived from the justice of God, which is supposed to be farther interested in the farther punishment of the vicious, and reward of the virtuous. But these arguments are grounded on the supposition, that God has attributes beyond what he has exerted in this universe, with which alone we are acquainted. Whence do we infer the existence of these attributes? It is very safe for us to affirm, that, whatever we know the deity to have actually done, is best; but it is very dangerous to affirm, that he must always do what to us seems best. In how many instances would this reasoning fail us with regard to the present world? But if any purpose of nature be clear, we may affirm, that the whole scope and intention of man’s creation, so far as we can judge by natural reason, is limited to the present life. With how weak a concern, from the original, inherent structure of the mind and passions, does he ever look farther? What comparison, either for steddiness or efficacy, between so floating an idea, and the most doubtful persuasion of any matter of fact, that occurs in common life. There arise, indeed, in some minds, some unaccountable terrors with regard to futurity: But these would quickly vanish, were they not artificially fostered by precept and education. And those, who foster them; what is their motive? Only to gain a livelihood, and to acquire power and riches in this world. Their very zeal and industry, therefore, are an argument against them. What cruelty, what iniquity, what injustice in nature, to confine thus all our concern, as well as all our knowledge, to the present life, if there be another scene still awaiting us, of infinitely greater consequence? Ought this barbarous deceit to be ascribed to a beneficent and wise being? Observe with what exact proportion the task to be performed and the performing powers are adjusted throughout all nature. If the reason of man gives him a great superiority above other animals, his necessities are proportionably multiplied upon him. His whole time, his whole capacity, activity, courage, passion, find sufficient employment, in fencing against the miseries of his present condition. And frequently, nay almost always, are too slender for the business assigned them. A pair of shoes, perhaps, was never yet wrought to the highest degree of perfection, which that commodity is capable of attaining. Yet is it necessary, at least very useful, that there should be some politicians and moralists, even some geometers, historians, poets, and philosophers among mankind. The powers of men are no more superior to their wants, considered merely in this life, than those of foxes and hares are, compared to their wants and to their period of existence. The inference from parity of reason is therefore obvious. On the theory of the soul’s mortality, the inferiority of women’s capacity is easily accounted for: Their domestic life requires no higher faculties either of mind or body. This circumstance vanishes and becomes absolutely insignificant, on the religious theory: The one sex has an equal task to perform with the other: Their powers of reason and resolution ought also to have been equal, and both of them infinitely greater than at present.  As every effect implies a cause, and that another, till we reach the first cause of all, which is the Deity; every thing, that happens, is ordained by him; and nothing can be the object of his punishment or vengeance. By what rule are punishments and rewards distributed? What is the divine standard of merit and demerit? Shall we suppose, that human sentiments have place in the deity? However bold that hypothesis, we have no conception of any other sentiments. According to human sentiments, sense, courage, good manners, industry, prudence, genius, &c. are essential parts of personal merit. Shall we therefore erect an elysium for poets and heroes, like that of the antient mythology?3 Why confine all rewards to one species of virtue? Punishment, without any proper end or purpose, is inconsistent with our ideas of goodness and justice; and no end can be served by it after the whole scene is closed. Punishment, according to our conceptions, should bear some proportion to the offence. Why then eternal punishment for the temporary offences of so frail a creature as man? Can any one approve of Alexander’s rage, who intended to exterminate a whole nation, because they had seized his favourite horse, Bucephalus?4 Heaven and hell suppose two distinct species of men, the good and the bad. But the greatest part of mankind float between vice and virtue. Were one to go round the world with an intention of giving a good supper to the righteous and a sound drubbing to the wicked, he would frequently be embarrassed in his choice,  and would find, that the merits and demerits of most men and women scarcely amount to the value of either. To suppose measures of approbation and blame, different from the human, confounds every thing. Whence do we learn, that there is such a thing as moral distinctions but from our own sentiments? What man, who has not met with personal provocation (or what good natur’d man who has) could inflict on crimes, from the sense of blame alone, even the common, legal, frivolous punishments? And does any thing steel the breast of judges and juries against the sentiments of humanity but reflections on necessity and public interest? By the Roman law, those who had been guilty of parricide and confessed their crime, were put into a sack, along with an ape, a dog, and a serpent; and thrown into the river: Death alone was the punishment of those, who denied their guilt, however fully proved. A criminal was tryed before Augustus, and condemned after full conviction: But the humane emperor, when he put the last interrogatory, gave it such a turn as to lead the wretch into a denial of his guilt. You surely, said the prince, did not kill your father.5 This lenity suits our natural ideas of RIGHT, even towards the greatest of all criminals, and even tho’ it prevents so inconsiderable a sufferance. Nay, even the most bigotted priest would naturally, without reflection, approve of it; provided the crime was not heresy or infidelity. For as these crimes hurt himself in his temporal interests and advantages; perhaps he may not be altogether so indulgent to them. The chief source of moral ideas is the reflection on the interests of human society. Ought these interests, so short, so frivolous, to be guarded by punishments, eternal and infinite? The damnation of one man is an infinitely greater evil in the universe, than the subversion of a thousand million of kingdoms. Nature has rendered human infancy peculiarly frail and  mortal; as it were on purpose to refute the notion of a probationary state. The half of mankind dye before they are rational creatures. III. The physical arguments from the analogy of nature are strong for the mortality of the soul; and these are really the only philosophical arguments, which ought to be admitted with regard to this question, or indeed any question of fact. Where any two objects are so closely connected, that all alterations, which we have ever seen in the one, are attended with proportionable alterations in the other; we ought to conclude, by all rules of analogy, that, when there are still greater alterations produced in the former, and it is totally dissolved, there follows a total dissolution of the latter. Sleep, a very small effect on the body, is attended with a temporary extinction; at least, a great confusion in the soul. The weakness of the body and that of the mind in infancy are exactly proportioned; their vigor in manhood; their sympathetic disorder in sickness; their common gradual decay in old age. The step farther seems unavoidable; their common dissolution in death. The last symptoms, which the mind discovers, are disorder, weakness, insensibility, stupidity, the forerunners of its annihilation. The farther progress of the same causes, encreasing the same effects, totally extinguish it. Judging by the usual analogy of nature, no form can continue, when transferred to a condition of life very different from the original one, in which it was placed. Trees perish in the water; fishes in the air; animals in the earth. Even so small a difference as that of climate is often fatal. What reason then to imagine, that an immense alteration, such as is made on the soul by the dissolution of its body and all its organs of thought and sensation, can be effected without the dissolution of the whole? Every thing is in common between soul and body. The organs of the one are all of them the organs of the other. The existence therefore of the one must be dependent on that of the other. The souls of animals are allowed to be mortal; and these bear so near a resemblance to the souls of men, that the analogy from one to the other forms a very strong argument. Their bodies are not more resembling; yet no one rejects the arguments drawn from comparative anatomy. The Metempsychosis is therefore the only system of this kind, that philosophy can so much as hearken to.6 Nothing in this world is perpetual. Every being, however seemingly firm, is in continual flux and change: The world itself gives symptoms of frailty and dissolution: How contrary to analogy, therefore, to imagine, that one single form, seemingly the frailest of any, and from the slightest causes, subject, to the greatest disorders, is immortal and indissoluble? What a daring theory is that! How lightly, not to say, how rashly entertained! How to dispose of the infinite number of posthumous existences ought also to embarrass the religious theory. Every planet, in every solar system, we are at liberty to imagine peopled with intelligent, mortal beings: At least, we can fix on no other supposition. For these, then, a new universe must, every generation, be created, beyond the bounds of the present universe; or one must have been created at first so prodigiously wide as to admit of this continual influx of beings. Ought such bold suppositions to be received by any philosophy; and that merely on pretence of a bare possibility? When it is asked, whether Agamemnon, Thersites, Hannibal, Nero, and every stupid clown, that ever existed in Italy, Scythia, Bactria, or Guinea, are now alive; can any man think, that a scrutiny of nature will furnish arguments strong enough to answer so strange a question in the affirmative? The want of arguments, without revelation, sufficiently establishes the negative. Quanto facilius, says Pliny,7 certiusque sibi quemque credere, ac specimen securitatis antigenitali sumere experimento. Our insensibility, before the composition of the body, seems to natural reason a proof of a like state after its dissolution. Were our horror of annihilation an original passion, not the effect of our general love of happiness, it would rather prove the mortality of the soul. For as nature does nothing in vain, she would never give us a horror against an impossible event. She may give us a horror against an unavoidable event, provided our endeavours, as in the present case, may often remove it to some distance. Death is in the end unavoidable; yet the human species could not be preserved, had not nature inspired us with an aversion towards it. All doctrines are to be suspected, which are favoured by our passions. And the hopes and fears which give rise to this doctrine, are very obvious. It is an infinite advantage in every controversy, to defend the negative. If the question be out of the common experienced course of nature, this circumstance is almost, if not altogether, decisive. By what arguments or analogies can we prove any state of existence, which no one ever saw, and which no wise resembles any that ever was seen? Who will repose such trust in any pretended philosophy, as to admit upon its testimony the reality of so marvellous a scene? Some new species of logic is requisite for that purpose; and some new faculties of the mind, which may enable us to comprehend that logic. Nothing could set in a fuller light the infinite obligations, which mankind have to divine revelation; since we find, that no other medium could ascertain this great and important truth.