It's also really funny to read economics papers from the early 20th century, because the field hadn't gotten comfortable with math yet. The following (from Tinberger, 1939, page 15) would be downright insulting to explain to a first year graduate student nowadays. I guess that's progress!
If we are to understand the mechanism as a whole, we must continue this procedure until the number of relations obtained equals the number of phenomena the course of which we want to explain. We should not be able to calculate, say, n variables if we had only n-2 or n-1 relations; we need exactly n. Such a system of as many relations as there are variables to be explained may be called a complete system. The equations composing it may be called the elementary equations. The word "complete" need not be interpreted in the sense that every detail in the complicated economic organism is described. This would be an impossible task which, moreover, no business-cycle theorist has ever considered as necessary. By increasing or decreasing the number of phenomena, a more refined or a rougher picture or "model" of reality may be obtained; in this respect, the economist is at liberty to exercise his judgment. A conclusion about the character of cyclic movements is, however, possible only if the number of relations equals the number of phenomena (variables) included. (The remark may be made here that there is no separate or special variable representing "the cycle" which has to be included in the elementary relations. It is by the mechanism itself that all variables included are compelled to perform cyclic changes.)Teehee! N independent equations solve for N unknowns, and dependent variables can't be independent variables. I'll keep that in mind :)