In order to see what is meant by the "third Heaven," one has in the first place to perceive what I desire to express by this word Heaven alone: and then one will see how and why this third Heaven was needful to us.

I say that by Heaven I mean Science, and by the Heavens "the Sciences," from three resemblances which the Heavens have with the Sciences, especially by the order and number in which they must appear; as will be seen by discussing that word Third.

The first similitude is the revolution of the one and the other round one fixed centre.

For each movable Heaven revolves round its centre, which, on account of its movement, moves not; and thus each Science moves round its subject, which itself moves not; for no Science demonstrates its own foundation, but presupposes that.

The second similitude is the illumination of the one and the other.