Extra Infusions?

[Tetraplex]

Hello all, I have a quick, probably dumb question that I can't seem to find a straight answer for anywhere.

Artificers get extra infusions if they have a high intelligence. They can also cast any infusion with a level equal to or lesser than their INT bonus. So can a 1st level Artificer with 20 INT (From 18 base with a favorable race) use one 5th-level Infusion per day? I would assume no, but I can't find anywhere that says I can't...I must be missing something, right?

[Fax Celestis]

A — in an ability is a nonability, or null. Null is different from 0: zero can be added to; null cannot.

Null + 1 is still null.

So where the fifth level infusions cell on the first level of the artificer table says —, it means that even if your artificer has a 20 INT, they're getting a bonus infusion to a null-value ability.

[Tetraplex]

Even though it says the Artificer knows them all? Ok, I guess I'll have to be content with 4/day on the 1st-level ones, then. Thank you!

[Fax Celestis]

Even though it says the Artificer knows them all? Ok, I guess I'll have to be content with 4/day on the 1st-level ones, then. Thank you!

He knows all his infusions just fine, but he doesn't have the ability to cast them yet. Ability to cast and knowledge of are two different things: a wizard can scribe a 5th level spell into his spellbook at 1st level, but he can't prepare it or cast it.