There is, or rather 'was', 'logic' to it at its conception, given the means available at the time. The scale is based on an upper and lower reference temperature, divided up into one hundred degrees. The lower temperature was an easy-to-establish 'zero', the freezing point of water. The upper reference was just as easy, being the temperature of a normal, healthy human, taken at the time to be one hundred. The scale is therefore from 0 to 100, freezing water to body temperature. Not too shabby for the early 1700's, no..? OK, they got it a little wrong with the 100, but with what they had to work with back then, it wasn't bad at all. There was, indeed, a lot of 'logic' to it.