The physical principle is that nothing exists but substance, which he makes of two kinds--the one a substance that thinks, the other a substance extended. Actual thought and actual extension are the essence of substance, so that the thinking substance cannot be without some actual thought, nor can anything be retrenched from the extension of a thing, without taking away so much of its actual substance. In his physical speculations, Des Cartes has allowed his imagination to run very wild. His famous theory of vortices is an example of this. Assuming extension to be the essence of substance, he denied the possibility of a vacuum by that assumption; for if extension be the essence of substance, wherever extension is, there substance must be. This substance he assumes to have originally been divided into equal angular particles, each endowed with an equal degree of motion; several systems or collections of these particles he holds to have a motion about certain equidistant points, or centres, and that the particles moving round these composed so many vortices. These angular particles, by their intestine motions, he supposes to become, as it were, ground into a spherical form; the parts rubbed off are called matter of the first element, while the spherical globules he calls matter of the second element; and since there would be a large quantity of this element, he supposes it to be driven towards the centre of each vortex by the circular motion of the globules, and that there it forms a large spherical body such as the suu. This sun being thus formed, and moving about its own axis with the common matter of the vortex, would necessarily throw out some parts of its matter, through the vacuities of the globules of the second element constituting the vortex; and this especially at such places as are farthest from its poles: receiving, at the same time in, by these poles, as much as it loses in its equatorial parts. And, by these means, it would be able to carry round with it those globules that are nearest, with the greater velocity; and the remoter, with less. And, further: those globules which are nearest the centre of the sun, must be smallest; because, were they greater, or equal, they would, by reason of their velocity, have a greater centrifugal force, and recede from the centre. If it should happen that any of these sun-like bodies, in the centres of the several vortices, should be so in-crusted and weakened, as to be carried about in the vortex of the true sun: if it were of less solidity, or had less motion than the globules towards the extremity of the solar vortex, it would descend towards the sun, till it met with globules of the same solidity, and susceptible of the same degree of motion with itself* and thus, being fixed there, it would be for ever after carried about by the motion of the vortex, without either approaching any nearer to, or receding from the sun, and so become a planet. Supposing, then, all