"If there are four equations and only three variables, and no one of the equations is derivable from the others by algebraic manipulation then there is another variable missing"

This quote by Talcott Parsons is referring to the principle of direct algebra. When handling direct formulas, there are a certain number of variables that need to be represented in order to fix the formulas. If there are 4 formulas and just 3 variables, then it is impossible to fix the equations without presenting another variable. This indicates that there is another variable missing out on from the equations, and it must be accounted for in order to fix the formulas. This quote is emphasizing the value of accounting for all variables when dealing with linear equations, as it is difficult to solve the formulas without them. It is likewise highlighting the value of algebraic manipulation when handling direct formulas, as it can help to identify any missing out on variables.