"He Calls it Thinking"

Response: Cancel the effects of a response, then attach this as a +2 str boon.

What if no response by my enemy was played? Can I not cancel the effects of a response and then attach it as a boon?

No, for 2 reasons.

The most important reason is that all Responses have play restrictions. If those play restrictions are not met, you cannot trigger the Response. For example, if a Response says "Response: after you win a challenge…," if you have not won a challenge, you have not met the play restrictions for the Response. If a Responses says "Response: after an opponent reveals a plot card…," if your opponent has not revealed a plot card, you have not met the play restrictions for the Response. More to the point, if you are trying to play a Response, you have to be responding to something. If that something hasn't happened, there is nothing to respond to, so you cannot play the Response.

When you are dealing with a "cancel" Response, you have to remember that cancel responses, by definition, interrupt the initiation and resolution of some effect - as defined in their text. So, when He Calls It Thinking says to cancel a Response without an influence cost, there is a play restriction involved, thanks to the definition of "cancel" effects, that says there must be a Response effect without an influence cost for you to interrupt. If no one has played a Response effect without an influence cost, the play restrictions for He Calls It Thinking are not met, and there is nothing to respond to or cancel. So you cannot trigger the effect.

Secondly, even if you could trigger it when there was nothing to cancel, you wouldn't get to attach it as a +2 attachment. That's because the effect says you "then" attach the card as a Boon attachment. When the word "then" is used in an effect, the part before the "then" must be successful before the part after the "then" can initiate. If you did trigger it without something to cancel, the cancel would be unsuccessful - and the "attach" part would never happen.

Thank you for your dedicated and continued patience.