Three writers of eminent reasoning power, but of widely different training and mental prepossessions, David Hume, Immanuel Kant, and Archbishop Whately, have maintained that the doctrine of the immortality of the soul is not capable of being demonstrated or logically deduced, from known facts.
Hume, sarcastically, and Whately, honestly, endeavour to prove that mens belief in immortality can only be justified by revelation, Kant postulates immortality as a necessity of practical reason; but all three agree in denying, that a belief in the immortality of the soul can be legitimately arrived at by those processes which lead to certainty in science.
Humes views are stated in his "Essay on the Immortality of the Soul," the first paragraph of which contains the following sentence; "But in reality it is the Gospel, and the Gospel alone, that has brought life and immortality to light." And the first of Whatelys essays "On Some Peculiarities of the Christian Religion" concludes with a warning against "unduly exalting Natural Religion at the expense of Revelation," and "underrating the value of the Gospel, and dishonouring Him who, through it, brought life and immortality to light." The fourth section of the second "Haupstuck" of the second book of the first part of Kants "Kritik der praktischen Vernunft" is headed, "The Immortality of the Soul as a Postulate of Pure Practical Reason." Kant argues thusThe perfect harmony of the affections with the moral law is the primary condition of the production of the highest good. This perfect harmony must, therefore, be possible. "But such perfect harmony of the will with the moral law is Holiness, a perfection of which no reasonable being of the world of sense is capable at any moment of its existence. Since, however, holiness is practically necessary, it can only be looked for in an infinite progress toward  that perfect harmony; and, therefore, according to the principles of practical reason, it is necessary to assume such a real progress as the real object of our volition. This infinite progress, however, is only possible on the assumption of the infinitely continued existence and personality of the same reasonable being, (which is called the immortality of the soul.) Therefore, the highest Good, practically, is only possible on the assumption of the immortality of the soul; consequently this, as being inseparably bound up with the moral law, is a Postulate of pure practical reason (by which I understand a theoretical proposition, as such, however, not demonstrable, so far as it is inseparably connected with an unconditionally obligatory practical law)."
Thus, for Hume and Whately the doctrine of the immortality of the soul is a revelation; for Kant, it is an assumption.
Humes arguments against the validity of the reasonings by which the immortality of the soul is attempted to be proved may be briefly summed up thus:
(a ) It is said that the soul is immaterial, and that it is impossible for thought to belong to a material substance. But matter and spirit are at bottom equally unknown, and for anything we know to the contrary, matter may be the cause of thought.
(b ) If an immaterial soul exists, the analogy of nature suggests that the immaterial substance of which it consists is constantly assuming new forms.
(c ) If an incorruptible soul exists, it must also be ingenerable. We have as much reason to believe that it existed before our birth, as we have to believe that it will exist after our death. But as we know nothing of the prenatal state of the soul, we have no reason to believe that our consciousness will be continued into its post-mortal state. And this is, practically, ceasing to exist.
(d ) The same arguments which prove the existence of souls in men prove their existence in animals. Are animals immortal?
(e ) It is said that the justice of God requires a future state of rewards and punishments. But our knowledge of the justice of God is limited by our experience of this world; and if His justice leads Him to inflict no other punishment  and bestow no other reward than that which we observe in the present state, there can be no proof that His justice requires anything more in accordance with our notions of justice, in a future state, supposing that future state to exist.
(f ) The faculties of men are said too be to great to find their full scope of this life. But not even a pair of shoes was ever as well made as it might be; and there is room for a practically infinite development of all mens powers here.
(g ) No argument from mere reason, or our ideas of justice, can prove that human offences deserve infinite and purposeless punishment. "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 the merits and demerits of most men and women scarcely amount to the value of either."
(h ) In the absence of proof to the contrary, the absolute dependence of the mental faculties upon the bodily organization is presumptive evidence that the former do not outlast the latter.
Whatelys main points are :
( a ) That before the introduction of Christianity the ancients had but a dim and confused idea of a future state.
(b ) That their reasons for the amount of belief they entertained were insufficient.
(c ) That the problem whether the soul is material or not is insoluble, inasmuch as we know nothing about the fundamental nature of mind or of matter. In fact, Whately adopts almost verbally Humes positions a, b, c, and d; and more generally he seems to agree with e and f.
(d ) Whately further argues that, even admitting there may be grounds for assuming a future existence, there are none for supposing it to be endless. That it is "extravagant" to suppose that even unmerited suffering in this world will be rewarded by an immortality of happiness; since a limited amount, say 1,000 years, of such happiness would, as a matter of justice, be ample compensation for any  quantity of such suffering. This is plainly obverse of Humes argument g.
(e ) The Archbishop fully adopts Warburtons conclusion, that the Pentateuch does not teach the immortality of the soul.
What Kant thinks of the power of the pure reasoning faculty of man to deal with such a problem as the immortality of the soul may be judged by the following passage from the "Prolegomena," taken in conjunction with that already cited:
"The position of all genuine idealists, from the Eleatic school down to Bishop Berkeley, is comprised in this formula: 'All knowledge through the senses and experience is nothing but empty figment, and truth lies only in the Ideas of the pure Understanding and Reason.'
"The fundamental proposition which rules and guides my Idealism throughout is, on the other hand, 'All knowledge of things by the mere pure understanding, or pure reason, is nothing but empty figment (lauter Schein ), and truth lies only in experience.'"
It follows, therefore, that Kant regarded the immortality of the soul as an hypothesis of immense moral value; to be assumed on account of that value, but in its very nature incapable of proof or of scientific evidence. Like Hume, he treats both Spiritualism and Materialism as errors.
I follow Hume, Kant, and Whately in defending the thesis that:
The Immortality of the Soul cannot be deduced by scientific methods of reasoning from the facts of physical or psychical nature.