The Lie rank condition for strong nonlinear accessibility is interpret
ed via the differential geometry of jets and prolongations of infinite
order. It yields to a new Lie algebraic criterion and to the consider
ation of first integrals, which apply to nonlinear systems in quite ge
neral form; the latter characterization in particular is valid without
distinguishing between the system variables and is independent of tim
e-scalings. Weak accessibility is also considered. (C) 1997 Elsevier S
cience B.V.