We construct reality in our heads, and many things do not hang together properly, as in our dreams - full of people, places, and occasions that do not fit together very well. We take great pleasure, when we finally succeed in assembling a picture, where everything seems just so. While a crossword or a jigsaw puzzle has just one correct answer, we often guess at how more complex things hang together - gossip is often about guessing from a few facts - and we never realise that we are wrong.

Scientists are always guessing at the big picture that might emerge from our scraps of evidence. We make mistakes - astrology was one of them - and we learn from these mistakes. But scientists usually doubt their constructions of their scattered facts. Or if they do not doubt their constructions, then their competitors are sure to do so. We keep trying to disprove a pretty picture, at the same time as we try to embellish it. Gradually, our construction of reality gets better underpinnings, and we become more sure that it is correct. Call this scepticism, or the search for truth - it is what gives proper scientific explanations their good reputation, in a world full of less coherent explanations.

William Calvin is author of books including A Brief History Of The Mind: From Apes To Intellect And Beyond (buy this book from Amazon (UK) or Amazon (USA)), and A Brain for All Seasons: Human Evolution and Abrupt Climate Change (buy this book from Amazon (UK) or Amazon (USA)). See his website.