Difference Between Rational and Rationale

‘Rational’ and ‘rationale’ are very similar words. They are spelled nearly the same and come from the same root word, which is the Latin ‘rationalis’. Additionally, their meanings are close, but they are not interchangeable because they are different forms of speech.

‘Rational’ is an adjective. Most often, it means that the thing it is describing is logical.

 “The belief that the earth is flat is not rational.”

In some cases, especially in discussions of intelligence, it means to have the ability to reason or use logic.

“Humans are rational creatures.”

When describing a person, it typically means that the person appears to be sane; their behavior is not illogical, contradictory, or just plain strange.

“I’m pretty sure that attacking someone with toothpaste is not a rational thing to do.”

In mathematics, it has a different meaning. A rational number is one that can be expressed as the ratio of two numbers. For example, the numbers 3, 5/6, and 0.5 are all rational because they can be expressed as ratios. The number pi, on the other hand, is not rational.

‘Rational’ can also be used as a noun to describe a rational number.

‘Rationale’ is a noun. Overall, it refers to the reasoning process itself. It can be used to mean an explanation of the reasons for a thought process or belief, or a review of the basis of it.

“The rationale for believing that he’s the murderer is that he was found with the murder weapon.”

In other cases, it can mean something that’s nearly the opposite: a justification for a thought process. This means that it is a set of reasons or an excuse made up to justify a position, as opposed to an explanation of a logical thought process.

“Her rationale for breaking into the house was that she thought it was being attacked by octopi.”

Oddly enough, both words have a common meaning that does not seem to be related to their primary meanings. They both refer to a type of breastplate worn as part of a priest’s robes. In this case, ‘rational’ seems to be the French spelling of the word and ‘rationale’, which is the original word, comes directly from Latin. The two of them are interchangeable here, so whichever one used will not matter much.

Overall, the two words should be easy to tell apart, especially in conversation. They are pronounced differently, especially at the end of the word. While the ‘ration’ part is pronounced the same, the second A in ‘rational’ is a short vowel, so it ends in an ‘ul’ sound. In ‘rationale’, the second A has a longer ‘ah’ sound, like found in the name ‘Al’.

It’s also easy to tell which is which just by seeing how it’s used. ‘Rational’ is primarily an adjective. It only has two noun uses, in mathematics and to describe the priest’s breastplate, and those should be obvious from context. ‘Rationale’ is only a noun. This is important to know in case there is a spellcheck error, where the spellcheck does not catch a problem because the word is a recognized word. It is also possible for a spellcheck to suggest both words as an option if there is a misspelling.

To summarize, the two words have the same Latin root, which is why they look similar. ‘Rational’ is most often an adjective describing something as logical or capable of reasoning, though it can also be used to describe certain numbers. ‘Rationale’ is always a noun and means either an explanation of a thought process or an excuse to justify something. Both can mean part of a priest’s clothing and they can be used interchangeably for that.

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