Let's start from the definition.
An astable (multi-vibrator) device is one unable to keep one state firmly but continuously oscillates between two states.
Up-down, one-zero, forth-back, all-nothing: whatever the way is in which you consider them the goal is the alternation between only two conditions.
By concentrating on the hardware we want to realize it: so where to start from?
High and low signals as referred to a ground one are furnished by every transistor and this means it's our first brick in the wall: let's say a very common one is enough for the purpose (for example the cheap BCxxx family bjts).
What kind of alternation is possible in output?
Not as the value of signals in itself but as the activation and control of the on-off.