I've never heard the following version of the ontological argument put quite in this way, though there may be things in Anselm and Descartes that suggest it:
- (Premise) An impossible being is imperfect.
- A perfect being is possible. (By (1))
- (Premise) Necessarily, a contingently existing being is imperfect.
- A perfect being exists necessarily. (By 2 and 3)
- (Premise) What exists necessarily also actually exists.
- A perfect being actually exists. (By 4 and 5)
Why believe (1)? Well, one line of thought is that impossibility is an impotence. Another is that an impossible property entails all properties, and in particular such properties as being imperfect, and no imperfect being is perfect.