Which principle does mathematician Legendre refer to in the quote: "Of all the principles that can be proposed for this purpose, I think there is none more general, more exact, or easier to apply than that which we have used in this work; it consists of making the sum of squares of the errors a minimum. By this method, a kind of equilibrium is established among the errors which, since it prevents the extremes from dominating, is appropriate for revealing the state of the system which most nearly approaches the truth."
