"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 concept of linear algebra. When dealing with linear equations, there are a certain number of variables that must be accounted for in order to solve the equations. If there are four equations and only three variables, then it is impossible to solve the equations without introducing another variable. This means that there is another variable missing from the equations, and it must be accounted for in order to solve the equations. This quote is emphasizing the importance of accounting for all variables when dealing with linear equations, as it is impossible to solve the equations without them. It is also highlighting the importance of algebraic manipulation when dealing with linear equations, as it can help to identify any missing variables.