I buy something because I have certain assumptions about how much happiness it will bring me.
Often assumptions are made which lead to equations in x which cannot be solved "rationally," i.e.
There is no subject of human study which may not be at some time or other of economic significance, and anything which affects the character, the ideals or the environment of man may make it necessary to modify our assumptions and our reasoning with regard to his conduct in economic affairs.
These assumptions, however, were fanciful.
In finding the abutment reactions some principle such as the principle of least action must be used, and some assumptions of doubtful validity made.