Speculation = creating new worlds = an alternate to reality = an imaginative journey.
So of course, speculation is crucial to the imaginative journey because it allows it to happen. The speculation furthermore allows for the individual to be self-enlightened (they love this concept apparently...about being self-enlightened).
If you think about it like that, it becomes an easier concept.
In relation to Coleridge, speculation in Lime-Tree = he speculates and conjures up in his mind what his friends are doing on their walk. To put it simply, he's annoyed that he can't go on the physical journey with him, but then he eventually realises that it's sometimes good to be denied pleasure as it's an uplifting experience...or something to that effect. The quote is "That we may lift the soul, and contemplate, with lively joy the joys we cannot share."
So yeah. Through the process of speculation he has gone on this imaginative journey and realised a positive.
