It may be that Keynes did not use the term "uncertainty" in the 
Treatise on Probability in this context, but he certainly had the 
concept and used the term later to relate to it.  Thus he identified 
four cases.  One is the straightforward one that Knight would call 
risk, and with Keynes not denying the frequentist element in it, even 
if that is not how he saw probabilities, which he defined at that 
time as being due to logical relations.  Flipping a coin is the 
canonical example.  Then he had a case that might now be labeled as 
one of a "mixed distribution," a combination of several probability 
distributions, possibly with weights.  This is linked to the matter 
of the weight of belief as well.  He then recognized where there may 
in fact at the logical level exists some probability distribution, 
but which observers will not be able to discern because of, well, 
epistemological reasons, inability to gather or estimate data or 
whatever.  Finally then one reaches the case where there is no 
probability distribution at all, which he later identified with the 
term uncertainty.  This analysis was actually carried out in his 
undergraduate thesis many years earlier than 1921.  Of course, he 
used uncertainty in connection with investment, but when that is not 
merely financial but real investment, it is closely linked to entrepreneurship.

It should be kept in mind that while Keynes is often labeled a 
subjectivist and lumped in with Bayesians, he did clearly state the 
objective existence of probability distributions, even if this were a 
Platonic existence, in the TOP, again with the coin flipping case an 
example.  People often simply ignore or forget this.  He is not so 
easily folded into the Bayesian camp, although he clearly viewed pure 
frequentism as having serious limits.

Ironically, given various people here asserting that Keynes was all 
about epistemological problems, Paul Davidson has long argued just 
the opposite, that for Keynes uncertainty was an ontological issue, 
and that for people like Knight and many others it was merely 
epistemological, that they were stuck in that third case of Keynes, 
although that may be unfair to Knight.

Barkley Rosser