Millar concedes that "It is not at all obvious how this change could have occurred", and he's a historical linguistics professor! Alas he doesn't expatiate it. Can someone please expound this semantic shift then?
It is not always easy to understand why a word changes its meaning. The word realize
formerly meant ‘make real’, and still sometimes does, as in She finally realized her childhood
ambition. But the word has acquired two new senses: ‘understand’, as in I realize
that time is short, and ‘come to understand’, as in She suddenly realized that she had
forgotten her keys. It is not at all obvious how this change could have occurred [Embolding mine], since the
new senses actually require a different construction (a that-complement clause) from the
old sense (a simple transitive construction). The change in meaning has been so dramatic
that few people are now aware that realize is related to real.
Revised by Robert McColl Millar, Trask's Historical Linguistics (2015 3e), pp 8-9. Trask died in 2004.
Best Answer
Selecting some of the OED's definitions in chronological order, I see a three step process (my numbering):
To give real existence to something.
To present as real to the mind; to make to seem real; to bring vividly or clearly to mind as if real
To conceive of as being real; to apprehend with the clearness or detail of reality; (in later use also) to become aware of or come to understand (a fact, situation, etc.).
In other words, the original meaning is to intentionally bring into reality; the second is to intentionally imagine that this were so; and, finally, to come to mind unintentionally
In the end, A is no longer one thing, but an idea/concept requiring a clause. A clause is required because what we are realizing becomes more complex, for example an entire situation, as the OED definition states in (3) above, which sums up this transformation in meaning. Meaning (2) is the fulcrum, making the process a mental one.
The that clause also nicely serves to distinguish (a) from (b) and (c):
(a) I realized my dream
(b) I woke up and realized it was a dream
(c) I realized it was a pipe dream.
It would be interesting to see whether we really can sharply divide all usages into (b) suddenly realize and (c) come to realize. We can usually tell from the context/timespan, but I think there are instances that blur the line, as, perhaps, in the following examples I've found.
I think the adjective first in these examples refers to the beginning of a gradual coming to realize process but also, perhaps, the first a series of discrete "realization events," which may each be more or less sudden, but, one would think, gradually less surprising.
Here IMO the slowly puts us in a no-man's-land (no-person's-land ?) between the eventive and stative senses as E.A. refers to them in his comment below. Would there be much difference if the time period were shorter by seconds or minutes (...and suddenly realized...), aside from clearly pushing the meaning to the eventive?