Our perception of time seems to become compressed as we age. A year was an eternity when I was young, and now they just fly by. Let’s assume this effect is real, and can be quantified. Now add an idea from neurobiology. I learned from Sam Harris that there is a non-zero time interval between the moment the brain arrives at a conclusion, or makes a decision, and the moment we perceive it. This is one of his main arguments in “Free Will”.

So, qualitatively, if there is a continued shortening in the perception of time as we age, and there is a non-zero time interval for us to perceive the results of our own thoughts, then, if we were immortal, at some point, our perception of that time interval would be less than the interval itself.

We would not notice the passage of time anymore, and thus effectively cease to be immortal. We may continue living, but we wouldn’t notice it!

I’ll do the math and come back later.