I'm sure that most of us have learned about the 7 characteristics of living beings at some point in biology. For those who aren't familiar with them, or simply don't remember them, do go and google them because I will post them so we can compare them with what we know of Titans.
1. Living beings are composed of cells
We can see that Titans have cells because of they can regenerate after being wounded. All living beings can regenerate and all living beings have cells.
2.Living beings have different levels of organisation
Titans have tissues and organs, namely bones, muscles, stomachs, and some sort of central control in their napes
3.Living things use energy
They are solar powered
4.Living things grow
Now this is where problems arise. We don't know if they grow, but if they do, then it is at huge rates, since there is a 12 meter difference between the smallest titans and the biggest ones (Colossal titan doesn't count).
5.Living things respond to the environment
Since they fall into comatose state when deprived of light and interact with humans, then this can be considered as a characteristic of the Titans
6.Living things reproduce
They have no visible sexual organs and if they reproduced asexually, that would mean that the offspring would be a clone of the parent, as far as I know. Titans are look VERY different from each other. So no, we have no proof that they reproduce.
7. Living things adapt to the surrounding environment
Do they even need to adapt at all? They don't bother anyone, save humans and shifters, and as far as we know nobody else bothers them, save humans and shifters. As far as we know they do not adapt.
Since they do not fulfill all the conditions mentioned above, we can assume that they are not living beings, but I still have doubts. What do you think?
Titans are a extreme form of body armor. an exoskeleton and machine so to say.
They are run by a dreaming human
They are just as alive as a car is with the driver sleeping behind the wheel
If this made sense.
Well but Armin noticed that after seeing where Eren came out of his titan, don't know if he told anybody yet about it...
To point 6. Reproduce:
Can't we call that, what the ape titan did, reproduce?
As for the human in the nape of the neck, yes what you said is quite plausible.
Mechs with no pilot?? Or a sleeping pilot (as in the case when Eren was meant to be lifting the bolder to block the hole in the gate, but right after shifting he was on auto-pilot as a mindless titan and almost killed Mikasa).
The fact that some titans seem to be able to speak is very mysterious to me. When Connie's 'mother' in titan form says 'welcome home'... That makes me think that there's a gap between being turned into a titan where the human mind is still in-tact enough for some of the original person to remain, and then you quickly lose all sense of self and your humanity as you become a mindless titan. In a sense, the human in the nape of regular mindless titans' necks is a passive / powerless vessel which is being used to 'power' the titan so it can be a mindless eating-machine.