“Reason is, and ought only to be the slave of the passions, and can never pretend to any other office than to serve and obey them.” That’s the 18th-century philosopher David Hume. I agree. I love my instincts — and, the more I read about decision science, the more sceptical I become about reasoned judgements. In business, I find, there are two kinds of decisions — good decisions and decisions that are easy to explain and defend. They are not the same. The use of reason massively weights people towards the second type. Unfortunately, the need to explain or defend a course of action may lead to a worse decision, because our natural instinct for arse-covering requires that we must then base our decision only on those few factors accessible to measurement or to numerical expression.