  The Standard Model is based on the gauge invariance principle with gauge
group U(1)xSU(2)xSU(3) and suitable representations for fermions and bosons,
which are begging for a conceptual understanding. We propose a purely
gravitational explanation: space-time has a fine structure given as a product
of a four dimensional continuum by a finite noncommutative geometry F. The
raison d'etre for F is to correct the K-theoretic dimension from four to ten
(modulo eight). We classify the irreducible finite noncommutative geometries of
K-theoretic dimension six and show that the dimension (per generation) is a
square of an integer k. Under an additional hypothesis of quaternion linearity,
the geometry which reproduces the Standard Model is singled out (and one gets
k=4)with the correct quantum numbers for all fields. The spectral action
applied to the product MxF delivers the full Standard Model,with neutrino
mixing, coupled to gravity, and makes predictions(the number of generations is
still an input).
