Often, an idea's meaning organically sprawls and blurs, and the resulting gain in the idea's flexibility more than offsets its loss of clarity. But when clarity is indispensable, unequivocal limits are required. A reasonable source of those sharp limits is the idea's verified implications. What are its distinctive relationships with other ideas? How does it affect human thoughts, observations, and actions? What are its intended contexts, because the importance of context shouldn't be underestimated? What must someone think and do to assess confidence in its accuracy? In essence, what is its total ascertained "overlap" with known realities, an overlap which might be complex, indirect, or abstract?
However, at first glance, such strict earthbound limits seem to be poorly suited to some ideas. For instance, goals are meaningful ideas precisely because goals don't match the deficiencies of known realities. And the mismatches are essential for illuminating which actions are necessary to enact the goal. Therefore an inflexible enforcement of narrow-minded limits would incorrectly classify goals as meaningless, due to the fictional aspects.
Unsurprisingly, the solution to the dilemma is to be more realistic by acknowledging the nuances of meaning. Even when meaningfulness is carefully pinpointed, it might belong at neither extreme. Its place might be in the middle of the long gradient between nonsensical and obvious. Indeed, the middle usually includes the interesting ideas, including goals. Each prospective goal varies in meaningfulness depending on the magnitudes of its relevant details.
To be specific, the meaningfulness of goals could vary by feasibility, i.e. the goal's overlap with the expected results of one or more definite plans. Or it could vary by ordinariness, i.e. the goal's overlap with typical contents and events. Or it could vary by salience, i.e. the goal's overlap with human desires. Or it could vary by presumption, i.e. the goal's overlap with shaky unproven premises of any kind. Or it could vary by modesty, i.e. the goal's overlap with circumstances which exist already.
By these measures, goals with greater aggregate meaningfulness could be termed "ideals". Goals with lesser aggregate meaningfulness could be termed "fantasies". So an extremely improbable goal, or a goal that's dependent on extremely improbable conditions, is closer to a fantasy than an ideal. A goal that fundamentally alters numerous things simultaneously is closer to a fantasy than an ideal. A goal that's based on mysteries is closer to a fantasy than an ideal. A goal that disdains immediate needs is closer to a fantasy than an ideal. A goal filled with unknown midpoints is closer to a fantasy than an ideal.
Of all the hints that distinguish ideals from fantasies, the quickest is the goal's origin. Ideals are more likely to start with known realities and then incorporate diligently considered tweaks. Fantasies are more likely to start with ethereal paradises and then assume that desperate effort will be enough to bridge the gap. Ideals are more likely to start by examining planet Earth in its current form. Fantasies are more likely to start by creating an imaginary alternative. Ideals are more likely to start with estimates of impacts and costs. Fantasies are more likely to start with nonnegotiable ultimatums.
The plain tactic of anchoring meaningfulness to solid referents doesn't eliminate amazing visions of better human lives; to the contrary, it's a fine way to choose the worthier ones.