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points in space, and at the same time a locus of forces having 
their seats in those material points. Let us suppose that 
the Author of the Universe, or some lower power competent 
to do it, were to remove all the matter from this locus, 
and leave a system of force exactly filling the locus of the 
removed matter, each force having its seat not in a point of 
matter hut in a point of space : suppose that these pure force- 
points in pure space should present the same resistance to 
my hand or my tools, should receive the same vibrations from, 
and communicate the same vibrations to, all that surrounds 
the locus, as the force did before the matter was removed. 
How could our senses detect the absence of the vanished 
matter? The table would feel and look the same, sound, 
smell, cut, burn the same, as before. Then it appears that 
our senses help us to no evidence either of the presence or 
the absence of this mysterious matter. 
Do you appeal next to the mathematician ? You refer to 
his moments, moving forces, living forces, &c., in all which 
the mass is before us in his formulae. It is true that the 
letter m is there ; but to the mathematician it is in all cases 
simply a number, a constant obtained by experiment only, 
experiment which is independent of all hypothesis about the 
existence or non-existence of matter. Newton, in the opening 
of his Principia, says, Virium causas vel sedes non expendo. 
He neither troubled himself about the cause, nor about the 
seat of force. It was sufficient for him to have the exact 
positions in space of the centre and points of departure of 
his forces with their directions and the numerical constant, 
given by experiment, which determine their intensities. 
No mathematician cares anything about material seats for his 
forces : he throws the matter invariably away, he contents 
