[tom_mellior]

I didn't say that the proof assistants were first order! I'm well aware that that's not the case. But AFAIK all the automation they use is first order. Higher order proof steps like "do this by induction on n" are always introduced by hand (at least in Coq and Isabelle), even if the rest of the proof is automatic.

[JohnStrange]

I don't understand what you mean by "all the automation they do is first order". They are based on higher order proof theories, e.g. higher order tablaux, and they implement higher order unification. Otherwise they would be first-order provers and thus much more limited.

But Yes, most common higher-order provers are semi-automatic, you need to give them a hint about which proof strategy to use. That's mainly because they are used that way, not any principal limitation. You won't find many mathematicians who are interested in a theorem prover to spit out some (alleged) theorem by itself, and then let the mathematician check whether it's useful.

The only fully automatic higher-order theorem prover that I know of is ETPS, it will select proof strategies by itself if you don't indicate them. But it's also one of the oldest and slowest and mainly just used for teaching logic.

[tom_mellior]

> I don't understand what you mean by "all the automation they do is first order".

I meant that the tactics of Coq that do reasoning for you, and the internal/external provers of Isabelle, are first order. I was probably partly wrong: you are right that some of them do use higher-order unification. But when in Coq I use "auto" or "omega" or whatever tactic to solve a goal, no higher-order tableaux are in use as far as I know. I have to massage the goal until I get it into a form that is palatable to the first-order automatic provers. Similarly, when I write an Isabelle proof like "from A have B by X; from this have C by Y; hence D by Z", the proof methods X, Y, Z are first order, often off-the-shelf SMT provers. Alternatively, there are also some built-in methods that use simple equational reasoning with higher-order unification, yes.

Let me know if I'm wrong about the details of this! Anyway, none of this means that you cannot prove complex higher-order stuff in these systems. You just can't do it automatically.

And, coming back to the start of this subthread, I don't think Mizar is really different in this regard.