Where do my known spells come from?

Question

I'd like to take some levels as a Wizard; in-game, the Wizard in my party teaches me some magic so that I can become a practicing Wizard. But, I haven't come up with a reason why I don't have exactly the same spells as my teacher.

In fact, I am not 100% sure how I get new spells as a Wizard. When I gain a level as Wizard I can add two new spells to my spellbook. For me, this brings up a lot of questions.

Do I have to find these spells first, for example on a scroll or in another spellbook? Would I need to pay 50 gp and spend 2 hours learning spells, per spell level? To me, that doesn't make any sense. So, I assume that I just pick two new spells with each level and additionally I can add the spells I find to the spellbook.

Do I “invent” the spells, e.g. “I would really like to be invisible, so I'll figure out a spell that makes me invisible”? I think that'd be an odd way to go about it. Perhaps it goes more like, “Oh, I remember that one afternoon when my mentor tried to explain how Fireballs work; now I finally understand it, so I can cast Fireball from now on.”

Summary Question: Where do the two new spells come from?

Answer 1

Your spells come from research, experimentation, and epiphanies as to the Weave of magic.

The boxed text on PHB pg 114 has a bit of flavor as to why this is so:

The spells that you add to your Spellbook as you gain levels reflect the Arcane Research you conduct on your own, as well as intellectual breakthroughs you have had about the nature of the Multiverse...

PHB pg 112 gives you more context to use for your flavoring of your dive into the Arcane arts of a Wizard multiclass:

...They learn new spells as they experiment and grow in (arcane) experience...

Did you find a mentor to teach you your first few spells? Do you carry around a very old Tome of spells you have slowly started mastering page by page? Did you decide "Hm..I'd really like to call down a Meteor Swarm one day." And when you finally figure out the magic involved you trademark it and start teaching all the younger wizards 'Airatome's Volatile Meteor Swarm' because the spell now exists because you created it?

That's all up to you.

(To directly comment on your In game reason.... keep in mind the moment you get a spellbook as a Wizard you can, actually, copy the spells your original party wizard knows from his own spellbook if he and the DM allow it. Doesn't mean you have to use them...just means on top of your level 1 spell allotment, you can also copy what he/she knows when you are of sufficient level to do so. Your in game reasoning for using different spell loadouts is because while your party wizard started you down the path to magic, you found your own 'School' to conduct your own studies in to.)

Answer 2

Rules-wise, see this question for an explanation.

In-game, it's up to you and your DM to decide where those spells "came from". You might decide that your character had observed these spells being cast and "derived" their workings, or that they had been researching those spells all along, or that they were always in your character's spellbook but your character didn't fully understand them until now. Or you can just not provide an explanation and just write the spells down.

Answer 3

Well, where does the other PC wizard's spells come from? Presumably not mailed to them from their Master.

In the AD&D Dungeon Master's guide, DMs are advised that Magic-Users (now called Wizards) will need to do one of three things to gain more than the one spell per level they received in that edition: independent research, trade spells with other [wizards], or find them in treasure hordes. The older edition also had a training requirement between levels, so I always assumed (in re-reading now I see no supporting evidence for this) that during level training the [wizard] was researching new spells, which would ultimately become the ones they learned when they achieved a new level.

The original DMG (OG DMG?;)) also points out that PC [wizards] will naturally trade spells, but that NPC wizards will jealously guard their spells from PC wizards. Perhaps your wizard then traded a spell with another wizard they met at a bar or library? As an aside, this is actually the second advantage of multiple PC wizards (beyond the obvious extra spells slots per day for the party) is that each wizard brings a more diverse spell pool to the group. Where before you were limited to two spells per level, now you get to choose four, which brings more options to the group.

If your DM levels you up around the time you find a big treasure horde (at the end of an adventure) maybe they can work in that you find a spellbook and copy the spells from there. As a DM I often use spellbooks as treasure.

In the 3rd Edition Forgotten Realms supplement "Magic of Faerun" presented the mage guild as a source of new spells, adding extra spells per level at the cost of membership dues (role-played and mechanical). City of Splendours (also a Forgotten Realms supplement) presents some other "Mage Guilds" which provide extra spells known per level. These are presumably the result of access to Guild owned spellbooks and the Guild's well stocked research library. In my own games, I push the Guild membership on my PCs to justify how they learn new spells if they don't come up with something better.

TL;DR wizards can explain learning nEw spells each level by a variety of means, including but. It limited to independent research, finding them in old tomes, copying them from the Wizard's Union local library, or swapping them with other spellcasters they meet in their travels.