But if force be granted this substantial and self-dependent character, what further need is there of matter as a separate form of entity? For does not the presence of matter consist essentially in resistance, itself a case of force? Such reflections as these led Boscovich and others to the radical departure of defining material particles _as centres of force_. [Sidenote: The Development and Extension of the Conception of Energy.] § 108. But a more fruitful hypothesis of the same general order is due to the attention directed to the conception of _energy_, or capacity for work, by experimental discoveries of the possibility of reciprocal transformations without loss, of motion, heat, electricity, and other processes. The principle of the conservation of energy affirms the quantitative constancy of that which is so transformed, measured, for example, in terms of capacity to move units of mass against gravity. The exponents of what is called "energetics" have in many cases come to regard that the quantity of which is so conserved, as a substantial reality whose forms and distributions compose nature. A contemporary scientist, whose synthetic and dogmatic habit of mind has made him eminent in the ranks of popular philosophy, writes as follows: "Mechanical and chemical energy, sound and heat, light and electricity, are mutually convertible; they seem to be but different modes of one and the same fundamental force or _energy_. Thence follows the important thesis of the unity of all natural forces, or, as it may also be expressed, the 'monism of energy.'"[236:8]