The immaterial of materials

I recall a lecture I once attended as a first year design student on the selection of metals for design projects. It was memorable in the way it was delivered, and the key tenet of the lecture remains still. 'In all cases use steel', the lecturer said before gesturing to leave the room . 'oh, except if you need it to be lightweight, then you should use aluminium'. He then left the room leaving my fellow students and me somewhat dumbfounded. Was that itda supposedly hour-long lecture reduced to one sentence? The lecturer returned after just long enough for the idea to sink in. He continued the lecture, 'oh, unless you need it to be really lightweight, and then you use magnesium', and then outlined more exceptions to this simple 'rule' of materials selection. The lecture had a profound effect on me. It was certainly an invaluable way of thinking about materials selection, but it left me insatiably curious about the choices we make when selecting materialsdwas that all there was to it? Do not get me wrong, I am in no way critical of the lecture or its messagedfar from it, for the simple elegance of this approach to selecting materials based upon their performance characteristics relative to their cost has helped construct the way I think about and select materials - but the thought that there must be more to material selection, and at what cost we select materials has remained with me ever since.