From The Screwtape Letters—a fictional work written from a senior demon’s perspective, advising a junior tempter.

It follows then, in general, and other things being equal, that it is better for your patient to be filled with anxiety or hope (it doesn’t much matter which) about this war than for him to be living in the present. But the phrase ‘living in the present’ is ambiguous. It may describe a process which is really just as much concerned with the Future as anxiety itself. Your man may be untroubled about the Future, not be cause he is concerned with the Present, but because he has persuaded himself that the Future is going to be agreeable. As long as that is the real cause of his tranquillity, his tranquillity will do us good, because it is only piling up more disappointment, and therefore more impatience, for him when his false hopes are dashed. If, on the other hand, he is aware that horrors may be in store for him and is praying for the virtues, wherewith to meet them, and meanwhile concerning himself with the Present because there, and there alone, all duty, all grace, all knowledge, and all pleasure dwell, his state is very undesirable and should be attacked at once. Here again, our Philological Arm has done good work; try the word ‘complacency’ on him. But, of course, it is most likely that he is living in the Present’ for none of these reasons but simply because his health is good and he is enjoying his work. The phenomenon would then be merely natural.

The Screwtape Letters
C. S. Lewis