Reasonable or Rational?

Last week we looked at the word reason but ran out of space before we could get to other forms, or to get to the question why reasonable has an s and rational has a t. 

Reasonable is the adjective form of the noun reason. It developed in about 1300 (it is reasonable to wonder what they used for an adjective for 100 years) but came from the Old French word raisonable which they got from the Latin word rationabilis. Etymonline quotes Erich Fromm, who in 1968 wrote in “The Heart of Man”, “What the majority of people consider to be ‘reasonable’ is that about which there is agreement, if not among all, at least among a substantial number of people; ‘reasonable’ for most people has nothing to do with reason but with consensus.” I disagree slightly, because there is still a certain amount of reasoning to it.

Reasonable meaning “moderate in price” came into use by the 1660s. When I use reasonable in this sense, what I mean is that the price stands to reason.

Rational is also an adjective, and came into English in the late 1300s. It may have come from the Old French word racionel, directly from the Latin word rationalis, or a combination of both. It originally meant “pertaining to reason” but by the mid-1400s also meant “endowed with reason.” It has a closer meaning to the noun reason than reasonable.

Reasonable would be similar in meaning to "making sense" while rational is similar in meaning to "having sense." Sanity is involved in rational, but not necessarily in reasonable. One can be insane and still reasonable.  

One interesting thing about these words is that their Latin words are all forms of the Latin word ratio, from which we get our English word ratio. In Latin ratio means “reckoning, numbering, calculation; or business affair and procedure.” When ratio came into use in English in the 1630s it meant reason or rationale, but by the 1650s it developed the meaning of a relationship between two numbers, its most common English meaning today.

But the Latin meaning brings us to another related word, rationale. When it appeared in the 1650s it referred to an exposition of principles, a meaning it retains today. But its primary meaning today is “the fundamental reason or reasons serving to account for something,” a meaning it developed in the 1680s. It came to English from the Late Latin word rationale, which is (as I’m sure you reasoned out) the noun use of the neuter of the Latin word rationalis. The final e makes a big difference in meaning. One must be rational to have a rationale.

From reviewing the etymologies it seems obvious that in adopting words from Latin the Old French would change the “ti” that sounds like “sh” in current English into an “s”. I was not able to find confirmation for this being the reason behind this change, but it is reasonable to assume it to be the case if you use the rationale that I am rational.