In the history of mathematics and natural philosophy Jacobi's contribution to theoretical mechanics is known as a part of the higher calculus in the tradition of Lagrange: accepted as mathematically important, it was denied to have any substantial physical or philosophical relevance. His last lectures on analytical mechanics, given in 1847–1848, three years before his death, and not yet published, show this view to be no longer tenable. These lectures are the most detailed and genuine source on Jacobi's views concerning (...) the foundations of mechanics. In particular, his criticism of the principles of mechanics is unprecedented for its time and anticipates some of Poincaré's views, published about fifty years later. The prehistory of Jacobi's Analytical Mechanics, a description of its content in general and an outline of his criticism, are presented in this paper. (shrink)