He, Ron, and Hermione spent the whole of break speculating on what Dumbledore would teach Harry.
The nature of that prophecy is unknown, although speculation is rife that it concerns Harry Potter, the only person ever known to have survived the Killing Curse, and who is also known to have been at the Ministry on the night in question.
There was no buzz of fear or speculation, however: Clearly, the news of Katie’s fate had not yet spread.
Now, you will remember, I hope, that I told you at the very outset of these meetings of ours that we would be entering the realms of guesswork and speculation?
There are no more uses of "speculate" in the book.
Show samples from other sources
philosophers have speculated on the question of God for thousands of years
We were speculating as to whether a president would have to resign after that kind of scandal.