I would say that an internal art is one where there is little or no conflict in the movement between tori and uke. In an external art, whilst the movement may essentially be the same as the internal, there is more conflict between tori and uke.
Or in another way:
External arts may use forceful blocks within their motion before redirecting/ countering an attack. Internal arts don't block, they try to absorb the attack keeping it in constant motion whilst redirecting it.
I appreciate that there is a lot more to this (strength vs power etc.), I'm not even sure that explains what I was trying to say, but that's as simple as I can put it.
Ultimately though, they both exist within each other, and can't exist without each other.
Just my quick thoughts over lunch