WHY IS THE MATRIX MODEL CORRECT

We consider the compactification of M theory on a lightlike circle as a limit of a compactification on a small spatial circle boosted by a l arge amount. Assuming that the compactification on a small spatial cir cle is weakly coupled type-IIA theory, we derive Susskind's conjecture that M theory compactified on a lightlike circle is given by the fini te N version of the matrix model of Banks, Fischler, Shenker, and Suss kind. This point of view provides a uniform derivation of the matrix m odel for M theory compactified on a transverse torus T-p for p = 0, .. .,5 and clarifies the difficulties for larger values of p. [S0031-9007 (97)04677-2].