# a confusing definition of “rationale”

I am confused about the definition of "rationale" and hope someone with more knowledge can share their insight.

Merriam-Webster defines "rationale" as

``````"an explanation or an underlying reason"
``````

It's very clear and easy to understand.

However, Oxford dictionary (https://en.oxforddictionaries.com) defines "rationale" as

``````"A set of reasons or a logical basis"
``````

The definition of "a logical basis" is similar to what Merriam-Webster has. But I have trouble with "a set of reasons". "A set of reasons" is countable in that there can be two sets of reasons. But generally speaking, which of the following sentences makes more sense ?

`"he explained the rationale behind the change"` Or

``````"he explained the rationales behind the change"
``````

Do we use the second sentence only when there are more than one set of "reasons", while the first one is restricted to only one set of "reasons" ? And what are "two sets of reasons" and what is "one set of reasons" anyway ?

• A thorough explanation usually includes multiple points. M-W is focusing on the multifaceted explanation treated as a composite, whereas ODO is emphasising the individual components treated one at a time. – Edwin Ashworth Mar 12 '18 at 22:23
• The rationale is a foundation - either a single reason or a structure, consisting of linked reasons. – Nigel J Mar 12 '18 at 22:35
• Is it correct that "rationales" are used only for an example like the following: the rationales for the "changes" - a rationale for 'change 1' and another rationale for 'change 2', where changes 1 & 2 are two distinct, independent changes that no one set of reasons can explain both? – B Chen Mar 13 '18 at 19:25
• B Chen, could you take Edwin's advice as a clue to how to combine the M-W and Oxford definitions? Either way, "rationales" as a plural is almost never used, and certainly doesn't help understand singular "rationale". – Robbie Goodwin Mar 27 '18 at 20:08