(Latin: in, not; mortalis, mortal)
Ordinarily understood as the doctrine that the human soul will survive after separation from the body, continuing in the perpetual possession of an endless conscious existence.
Together with the question of the existence of God, it forms from a practical point of view the most momentous question with which philosophy has to deal; for the practical attitude of a man towards the present life is necessarily affected by the position he takes respecting immortality.
Belief in a future life of some sort seems to have been almost universal at all times.
Immortality in the strict sense forms the foundation of the whole scheme of Christian faith.
The doctrine received its complete philosophical elaboration from Saint Thomas Aquinas in the 13th century.
Following his lead, Catholic philosophers, with few exceptions, have rightly maintained the demonstrability of immortality without appeal to revelation.
They argue on the one hand from the substantiality and especially spirituality of the human soul, on the other from man's innate natural desire of perfect happiness, and from an adequate sanction for the moral law.
The first line of argument alone can be summed up here.
The objects of the activity of the human intellect are intrinsically independent of matter and material conditions, being concerned with immaterial things, and with material things in universal (general) and intelligible ways, in ways independent of time and space; moreover the human intellect is capable of strict reflection, i.e., of turning about and as it were grasping itself, while material or corporeal agents always grasp something else than the agent.
Now activities such as these demand a subject or agent (the soul) that is intrinsically independent of matter and material conditions, an agent that is immaterial or spiritual.
Now an agent that is spiritual must be incorruptible, i.e., imperishable, and hence capable of surviving separation from the material conditions in which it exists in man, and be immortal.
And since the soul's faculties (intellect and will) remain rooted in the soul with the latter as an ever-present object, the immortal duration of the soul will be a vital or conscious existence.
New Catholic Dictionary