q & a from patents

This question and answer is from the Patent Law blog. It touched on some issues raised in the review session today, so I thought I'd cross-post it here.

I have a question regarding third party uses of an invention and the prior public use bar whenever you have the chance.

In my notes I have written that if a third party uses an invention in
secret and sells the output, this would likely not constitute prior
public use because the actual inventor had not commercially exploited
the invention himself. However, elsewhere in my notes, I have that if a
third party reduces an invention to practice and uses it, the patent of
the inventor is invalid (Baxter International). Would it be possible for
you to briefly explain third party activity in regards to prior public
use because I am a bit confused what type of third party activity
constitutes prior public use... In the first case it seems like the
third party is taking the invention and exploiting it, whether as the
second case it seems as though a third party is actually coming up with
the invention on his own, simultaneously with the inventor.

The key is that the commercial activity in the first case is (a) not actually of the invention, and (b) is kept secret, and thus doesn't constitute the sort of 'public' use we think of in these cases. In the second, case, the use is both (a) of the actual invention and (b) not secret (or 'public').