The End of Reductionism?

In one of Susskind's many YouTube lectures he wonders if reductionism has come to an end.

The closer you look at X's, the more they seem to be made of Y's. What are Y's made of? The closer you look at Y's, the more they seem to be made of X's.

You can't reduce further than X's and Y's. This looks like non-wellfounded (set theory) behavior. Somebody should look into this.