I often read that positive feedback is used to get oscillators (since the output of the circuit is summed up to its input signal and so leads to instability), while negative feedback to get amplifiers (since its output is subtracted to its input and so leads to stability).
But it seems to me that this intuitive reasoning is in contrast with the analysis of stability: in fact a negative feedback system is not always stable and a positive feedback system is not always unstable (we may check their stability by using the Nyquist criterion, for instance).
So why is it often used the first sentence?