Probabilism is the 16th century philosophical doctrine that certainty is unattainable but that belief and action must be governed by probability. As long as the existence, interpretation or application of a law remains doubtful, one may follow his own inclination, on the ground that a doubtful law cannot impose a certain obligation. But the subject “probabilistic design” embraces the idea of conceiving or planning of a thing so as to be fit for its purpose, more likely than not, but with less than absolute certainty. The designed “thing” is to be reliable. The lay meaning of “reliable” is trustworthy, predictable and good, and carries an implication that at successive points in time the “thing” will or could be somehow exposed to its purposed use and will successfully fit its purpose every time or most times.