Now that we have looked at all the lessons we can properly evaluate the cards with the learn mechanic. This evaluation is given based on being able to get any lesson whenever these cards trigger with no need to pick or include the lessons at all. The whole group of lesson and learn cards are basically unplayable without this proviso in cube. In a rotisserie style of event with lots of sideboard slots available you might consider a learn card and a couple of lessons but even then I doubt it. The power is just not up to that of most rotisserie decks.
Guiding Voice 0
Not doing enough for you. This has neither value or power or even utility. The learn doesn't make up for that. Not by a long way.
Professor of Symbology 8
Likely the best of the learn cards in a similar sort of way to Thraben Inspector being the best investigate card. It is a cheap body that works with flicker effects or buffs while also being just about relevant by itself. It can be a card neutral chump for slower decks or a free beater with some utility for aggro decks. Calling this the white Watcher for Tomorrow is a bit generous but it isn't too far off the mark.
Sparring Regimen 3
This is powerful but I fear let down by being narrow. You need a deck which attacks every turn to make this good. That limits this to very few archetypes. In those do you want this over an Anthem? An equipment? A proper value or removal card? Feels like the answer to all of these is no. Not enough punch for the places you want it and too few of those as well. The costlier the card gets the less value the learn is as a proportion of the card. As such the costlier the card the more you have to want the effect and I can't see that being the case for this one.
Study Break 2
The Expose Evil of learn cards! Not powerful enough nor widely useful enough. Give me the tapper that cycles over this thanks. With that or Expose Evil seeing no play this has little chance.
Arcane Subtraction 2
Much the same as Study Break. Too situational and too low impact.
Divide by Zero 5
This is pretty close to the mark but I think I prefer Repeal and Repulse. By and large it is dorks you are bouncing and so card draw wildly outdoes learning. Into the Roil might win out too as it has a two mana mode for those tempo plays. I like Divide a lot but three is a fairly big price to pay for bounce and the lessons you can get as a blue player don't get me very excited for generic over cost bounce leading into them.
Dream Strix 6.5
This is comparable to Cloudkin Seer. This has a bigger body for the cost but is learn rather than draw and only triggers on the death of the card. This is quite meaty and offers a decent amount of board control. It is one of the more proactive and tempo positive learn cards which is a plus but then blue has some of the less exciting options on things to find. As far as I can tell this card is pretty on the line and is somewhat of a filler dork you pad out a list with.
Pop Quiz 4
A kind of instant Divination. I like the blend of finding a card for the situation and also digging into the deck to hit lands and good stuff. Sadly this is just very low tempo and pretty low powered. I can't imagine instant Divination would get all much action in my cube these days. I would much rather try and get my value in tempo positive ways or in bigger chunks later down the line.
This is pretty comparable to Dream Strix. Just a flying dork that dies into learning. Sure, this is a lot less relevant but it also only costs one. I have marked a few of the learn cards as undesirable as they lacked power. While you could argue this of Eyetwitch a body is actually pretty good regardless of stature. Black likes to have things to sac, things to do with early mana, and things to discard. This is a bit Thraben Inspector and a bit Stitcher's Supplier. Just a filler one drop you can usefully ram into plenty of places.
Hunt for Specimens 6
This is basically Eyetwitch but with the learn upfront. Flying is better than the life and an actual body is probably better than a token producing spell in black. Black also has plenty of good two drops it can play. As such I expect this not to last despite being pretty close in many ways to a few cards that I expect to be the best of the learn spells.
Poet's Quill 3
The lifelink is appealing with black having some good ways to spend life. Still, this is mostly just a jumped up Leonin Scimitar with twice the costs and learn. Learning doesn't make up for a bad card and so this shouldn't be something people want to play.
Academic Dispute 3
Very cheap for a card that replaces itself but still not doing enough. Overly situational and not wildly effective.
Enthusiastic Study 2
A bit more useful than some of these learn cards but combat tricks want to be cheap. This is a clunky and poor combat trick as such and learn isn't offsetting the extra cost. I am not sure I would even run this if it was just draw a card instead!
First Day of Class 2
I mused this could be a Burning Wish style card. I was wrong. It can't. Lessons simply lack the range and power. As such this just falls into the camp of being bad compared to cheaper cards that do the same or better without learn. The effect can be used with persist dorks for infinite combos but this is more exciting for pauper than anything else.
Retriever Phoenix 5
Not enough cards with learn are going to be in the cube to trigger this recursion. As such this is just a 2/2 flying haste with learn. Not awful but far too low threat to really make the cut at that price range. Red does seem like it can benefit a little more from learning than some other colours and so I will give this a little test. It is proactive, utility, and value and so ticks a lot of cube boxes. I just feel that there are three mana versions of this type of card that offer more value as they will be more able to return themselves, and a 2/2 flying haste for 3 is better than the things you can learn! Just about worth a test but ultimately this is a capped value card at a mana cost where you are after raw power.
Igneous Inspiration 7.5
Volcanic Hammer that learns for one extra mana. While not the most efficient burn spell it is reasonably cost, fairly meaty, and still plenty playable. This seems like one of the best learn cards. Simple and direct. If this was draw a card it would be very exciting. I can imagine just tossing this into any old red deck and being pleased with the results.
Field Trip 5
This is competing with the likes of Cultivate and Llanowar Visionary. Three mana ramp and a two for one. This is low tempo and offers poor fixing (you can go via Environmental Sciences if you like..) but it is one of the better ramp spells to hit late. Three mana ramp has to go above and beyond for me and this just short in my high tempo cube. Moving towards slower, more value, or more budget cubes this seems to get significantly better but perhaps still not good enough. Especially with this kind of effect in green generally being a budget option.
Overgrown Arch 3
An interesting take on Wall of Blossoms or Wall of Mulch. The lifegain aspect is very interesting and makes this a much more appealing road block as far as handling aggression goes. It does also make it more valuable to remove for those players and with not assuring a card unless you have two mana spare when it is removed it seems a little risky. If you can't block with your wall for fear of it easily being answered and putting you a card down then your wall isn't really doing it's job. For a cheap stall card I would rather the no fuss up front original Wall of Blossoms. If I was desperate for defence and lifegain I might look to the likes of Jaddi Offshoot, or more likely than that some sort of more rounded Kitchen Finks. So while this is cool it looks like it is beaten out again by too many similar alternatives.
Gnarled Professor 7.5
This is likely the second or third best learn card. A big fat trampling beater that doesn't cost a card. Pretty simple. Passes the Doom Blade test and is sufficiently big that it will need answering in some way fairly promptly. Also late enough with enough pressure that you can easily put the lessons you find to good use while your opponent deals with your 5/4.
Rise of Extus 0
Removal needs to be cheaper than this. Quite substantially so. I wouldn't play this if it learned twice nor drew twice for that matter.
Cram Session 4
This is a kind of Revitalize. 4 life is meaningful and two mana is pretty cheap. This can also find some things that gain further life. Curve this into a pair of 1/1 pests and you have done a good amount of stalling and gained a pile of life. Viable game plan against some aggro lists. Most decks are not aggro however and then this is a do nothing that finds a bad card and as such is unlikely to be wanted main. It is also a bit underpowered for a sideboard card and thus probably won't get much action at all. Close again but just not quite.
That leaves us with 3 good learn cards and another 3 or so that are potentially enough for cube. Fewer than I expected when I first started seeing the mechanic spoiled.