@David Z, Not sure how much longer I can stay up... But could you give an example of where "Having haste and being not being a creature are identical in all practical aspects." is false wrt summoning sickness?

If X stops being creature, X stops being affected by SS. If X gains haste, X stops being affected by SS. If X starts being a creature, X resumes being affected by SS (if X still has it). If X loses Haste, X resumes being affected by SS (if X still has it).

Where's the practical difference?

@ikegami sorry, I had to take off for a while there

I would frame it like this, for example: a creature with haste can attack on the first turn you control it, but a noncreature permanent cannot. There's one difference.

What's that got to do with SS?

You could remove "on the first turn you control it" and it would still be true. This has nothing to do with SS.

Haste doesn't grant the ability to attack.

@David Z, [replied, but forgot to tag]

@ikegami the fact that it is true on any turn means that haste is not the same as not being a creature

Having haste and not being a creature both mean that a permanent is not subject to first-turn restrictions on its behavior which differ from the restrictions on subsequent turns, so they are the same in that one practical aspect, but that's not at all what I understood your comment to mean.

@David Z, I said haste is the same as not being a creature for the purpose of SS.

Why do I eve have to say that. You're not an idiot!

@David Z, I'm still waiting to hear of an aspect in which they differ.

As I said, a creature with haste can attack on the first turn you control it but a noncreature permanent can't. I guess you don't consider that a difference for the purpose of summoning sickness, but that wasn't clear from your wording.

@David Z, Unclear? You think it's possible I was saying a Hasted creature isn't a creature? You mustn't think much of me.

@David Z, Anything else?

I agree with @ikegami on this one. But I think you could better word it as "Haste makes a creature unaffected by summoning sickness like non-creature permanents" or something.

@murgatroid99, My wording was "Two exceptions: Hasted creatures and non-creatures are unaffected. (Both equally suppress the symptoms of summoning sickness. Remove the Haste or animate the permanent and they'll resuming having the symptoms.)"

I took shortcuts afterwards.

@ikegami Oh, that looks fine. The argument about "X for the purpose of/in the context of Y" just felt like familiar ground and I think it's better to be explicit and avoid potential ambiguity with interpretation

@ikegami I meant it wasn't clear that when you used the wording "for the purpose of summoning sickness" you meant to exclude that particular difference.

@ikegami Since the summoning sickness rule is specifically a rule about creatures, it's not strictly accurate to say that non-creature permanents have summoning sickness but are not affected by it, even if the conclusion is functionally equivalent

However, I think the most important thing to remember here is that "summoning sickness" does not actually have rules meaning, so this argument is primarily semantic. The important thing is to be clear about the conditions under which a permanent can or cannot attack and activate abilities with the {T} symbol.