How do I know if a lifetime of research actually mattered?

You have built something meant to outlast you, a research program that will keep going when you are gone. That is a real achievement, and yet the question sits there anyway: did a lifetime of it change anything? That you ask is not despair. It is the honest accounting of a serious person near the summit, looking back down the mountain. Here is a gentler truth than the question assumes. Change from work like yours is almost never a single visible event. It is slow and distributed, carried by students you taught, ideas that entered the water and lost your name, arguments settled quietly because of something you established. You are asking to see a harvest that was designed to be scattered, and scattered things are hard to count. Rather than trying to measure the whole, name one thing that is truer, or one person who thinks more clearly, because of your work. Let that stand as real. A life is not weighed all at once. It is enough to have moved the ground a little, in a direction that will keep moving after you.