You can't do without whatever it is you need, you can do without whatever it is you want.

I want to read that specific book but I need to be able to read.

At the most fundamentals, we need very little: food, shelter and then to be able to reproduce. But there are other 'needs', those would just be the true basic ones.

Needs are instrumental to some normative goal. You might have a reason to believe, do, or act. To that end, you have “needs.”

Desires are just natural facts, like headaches. Desires aren’t rational or irrational. They just are. The way you respond to your desires can be good or bad, but the desire itself is something that’s happening to you.

So, for instance, happiness might be one of your values. To that end, you have something like Maslow’s hierarchy of needs to worry about.

You also have to contend with a laundry list of desires, fulfilling some of which could make you happy or sad. Rationality is your sensitivity and responsiveness to facts that count in favor of acting on (or eschewing) these desires.

Those two are rather different, but a rule of thumb could be... a need, as the word implies, is necessary, to keep going or to achieve something else. A desire would be nice but you can live without it.

Mmm. I'd put them both in the spectrum of want, where need is simply further in, being more "pressing", a thirst that physically or psychologically "has to be quenched". Biological stuff like hunger and the need for sleep are here but I mean, you don't have to fulfill them because you can just die and it's not like the universe would crumble because of it. It's not "necessary".

And, whilst reason can help you understand these wants (and classify them and even curtail them), they come up by themselves. Like, even infants understand injustice and hate it and wanna "make things right" (YT has several videos on the topic), even if they don't have the words to express it yet.

Hopefully what I said here makes some sort of sense, lol.

Well, I almost always try to reason through my own actions, or have already done that reasoning and then come up with a set of guidelines.

To try to put them simply:

Will I likely die or become gravely ill or seriously injured or degrade my body or mind in an extreme manner or experience extreme pain without this thing or activity or... state of being?

If yes, then its a need.

Everything else is a want.

There are of course different tiers of wants and needs.

And, people are different, and some things may move up and down those tiers, and people will never fully agree on every different thing... also, addiction becomes a relevant concept to consider.

This one guy Maslow had some ideas about that once.

If I'd die without it, it's a need. Otherwise a desire.

Raised Catholic. Anything that makes me feel guilty is a desire.