"Without reason" or "Without reasons"?

Do we say "Without reason" or "Without reasons"? (e.g. She started laughing without (apparent) reason(s).).

Is "reason" countable or not?

Can we ever use a plural noun after "without"?

In the phrase "without reason", reason is a noun. Reasons can be counted so "without reasons" is grammatical. However, it is not idiomatic; the English idiom is to use "without reason". Note also that this is a formal usage which is uncommon in everyday conversation:

She laughed without reason (formal register)

contrasted with

She started laughing for no apparent reason (informal, everyday register)

reason most definitely can be countable: "give me five reasons why it isn't", but it can also be uncountable, "Reason is Man's greatest ally". Which are we dealing with? We can't quite pin it on without either; off the top of my head, "life is awful without friends" or "you can't do that without grounds to do so".

To my ear, making reason plural is unquestionably wrong.

*She started laughing without (apparent) reasons.

She started laughing without (apparent) reason.

I can't think of a single grammatical ground for invalidating the first. The fact that we can find other instances of without and a plural noun should prove it possible. Despite this,it is simply not said. I don't think you wouldn't be understood if you said it, but it would raise eyebrows and red pens. This isn't really an idiomatic phrase, perhaps in use, but not in the sense of compositionally; there is certainly something to be had from the meaning of just these words.

I am assuming here that we're after the without reason that means something like "without justification". The OED suggests that this meaning comes from the Middle French sans raison, from the Old French sanz reisun—the first recored use was in a1387. I'm no expert in French, but Google translates this as unjustifiably, which seems like a good start. I'd be interested to hear the modern version from a native speaker.

This construction is not without companions. Compare to:

without rhyme or reason

without reason or measure

without friendship/companionship, justification

There are other examples; these things do exist. It's almost as if these things, especially in the abstract, exist beyond count. Also compare, "she said it without justification" not *"she said it without justifications", but "she said it without grounds" not *"she said it without ground". In cases like without reason, the plural is unnecessary and muddles the meaning. I'm stil not entirely sure why–input?

"Reason" is certainly a count noun. However, it is also a verb. In the sentence "She started laughing without reason" could mean either "she didn't have a reason to start laughing", or "she starting laughing without thinking about it." In the first example, it is a noun, in the second it is a verb.

The most common way is to say "bla bla bla... without reason", and it is would probably be the countable noun, most of the time, but is slightly ambiguous. It would be perfectly fine for you to say, "she started laughing without reasons". It is grammatically correct, just not the most common way of putting it - which is probably a good thing.

So, to answer, your question: "Yes."

According to a COCA query americans say "without reason" 21 times more often than "without reasons". Statistically speaking She started laughing without reasons is odd. According to CALD reason is both countable and uncountable. Yes one can use a plural noun after "without". Again, a COCA query provides hundreds of hits for without followed by a plural noun.