I linked to this opaquely on Thursday, but it deserves its own entry. Michael Bierut’s 2005 essay called “On (design) bullshit” is one of my favourite design essays:

It follows that every design presentation is inevitably, at least in part, an exercise in bullshit. The design process always combines the pursuit of functional goals with countless intuitive, even irrational decisions. The functional requirements — the house needs a bathroom, the headlines have to be legible, the toothbrush has to fit in your mouth — are concrete and often measurable. The intuitive decisions, on the other hand, are more or less beyond honest explanation. These might be: I just like to set my headlines in Bodoni, or I just like to make my products blobby, or I just like to cover my buildings in gridded white porcelain panels. In discussing design work with their clients, designers are direct about the functional parts of their solutions and obfuscate like mad about the intuitive parts, having learned early on that telling the simple truth — “I don’t know, I just like it that way” — simply won’t do.

So into this vacuum rushes the bullshit: theories about the symbolic qualities of colors or typefaces; unprovable claims about the historical inevitability of certain shapes, fanciful forced marriages of arbitrary design elements to hard-headed business goals. As [Harry G.] Frankfurt points out, it’s beside the point whether bullshit is true or false: “It is impossible for someone to lie unless he thinks he knows the truth. Producing bullshit requires no such conviction.” There must only be the desire to conceal one’s private intentions in the service of a larger goal: getting your client it to do it the way you like it.

“I don’t know, I just like it that way” is such a tricky part of craft.