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Longitudinal stability. With the Centre of Pressure behind the Centre of gravity, the force from the tailplane must act downwards to counteract the pitch down moment. If the aircraft then pitches down, the (negative) angle of attack of the tailplane increases, so the downward force increases causing the nose to pitch back up. Converse is true for an initial pitch up. Thus the aircraft is longitudinally stable. If it deviates from a given pitch attitude, the tendency will be for it to return to that attitude.
Put the CoP ahead of the CoG and the force from the tailplane now has to act upwards. Sounds good - you now need a smaller wing. But if the aircraft pitches down, angle of attack of the tailplane increases, upward force on tailplane increases, aircraft pitches further down. So aircraft is longitudinally [U]unstable[U].
Unstable configuration can be made to work, but you need a very clever computer between the stick and the elevator!