From The Constitution of Liberty by F. A. Hayek
Humiliating to human pride as it may be, we must recognize that the advance and even the preservation of civilization are dependent upon a maximum of opportunity for accidents to happen. These accidents occur in the combination of knowledge and attitudes, skills and habits, acquired by individual men and also when qualified men are confronted with the particular circumstances which they are equipped to deal with. Our necessary ignorance of so much means that we have to deal largely with probabilities and chances.
The classical argument for tolerance formulated by John Milton and John Locke and restated by John Stuart Mill and Walter Bagehot rests, of course, on the recognition of this ignorance of ours. It is a special application of general considerations to which a nonrationalist insight into the working of our mind opens the doors. We shall find throughout this book that, though we are usually not aware of it, all institutions of freedom are adaptations to this fundamental fact of ignorance, adapted to deal with chances and probabilities, not certainty. Certainty we cannot achieve in human affairs, and it is for this reason that, to make the best use of what knowledge we have, we must adhere to rules which experience has shown to serve best on the whole, though we do not know what will be the consequences of obeying them in the particular instance.