  In this talk I first briefly review how far we have come in answering old
questions about the most fundamental building blocks of matter. I begin with
things we know, which is the Standard Model, and then talk about things we can
guess, which is superstring theory. After this review I discuss a key point at
which our understanding of superstring theory presently stops: the problem of
supersymmetry breaking and the cosmological constant. I explain in which
direction I imagine a way out. This way out predicts gravitinos and dilatons
with masses of order milli-eV.
