As everyone can easily understand, the interior life is an elevated form of intimate conversation which everyone has with himself as soon as he is alone, even in the tumult of a great city. From the moment he ceases to converse with his fellow men, man converses interiorly with himself about what preoccupies him most. This conversation varies greatly according to the different ages of life; that of an old man is not that of a youth. It also varies greatly according as a man is good or bad.
As soon as a man (woman) seriously seeks truth and goodness, this intimate conversation with himself tends to become conversation with God. Little by little, instead of seeking himself in everything, instead of tending more or less consciously to make himself a center, man tends to seek God in everything, and to substitute for egoism love of God and of souls in Him. This constitutes the interior life. No sincere man will have any difficulty in recognizing it. The one thing necessary which Jesus spoke of to Martha and Mary (1) consists in hearing the word of God and living by it.
The interior life thus conceived is something far more profound and more necessary in us than intellectual life or the cultivation of the sciences, than artistic or literary life, than social or political life. Unfortunately, some great scholars, mathematicians, physicists, and astronomers have no interior life, so to speak, but devote themselves to the study of their science as if God did not exist. In their moments of solitude they have no intimate conversation with Him. Their life appears to be in certain respects the search for the true and the good in a more or less definite and restricted domain, but it is so tainted with self-love and intellectual pride that we may legitimately question whether it will bear fruit for eternity. Many artists, literary men, and statesmen never rise above this level of purely human activity which is, in short, quite exterior. Do the depths of their souls live by God? It would seem not.
This shows that the interior life, or the life of the soul with God, well deserves to be called the one thing necessary, since by it we tend to our last end and assure our salvation. This last must not be too widely separated from progressive sanctification, for it is the very way of salvation.