Form follows function or function follows form?

Is it FORM follows function or FUNCTION follows form? Normally they both need to follow each other. Form need to follow function and function need to follow form. It is like making a plan to make a design by using form first followed by the function. Function also can be done first followed by the form to make it more efficient on the functionality. It really depends on the individual on which they want to use on making a design because by using any of them can create a perfect design.