Demur. You can conceive of a unicorn in the universe or system of mental objects, and it will exist quite happily there as long as you continue to so conceive.

When you get to the point where you can, simpply by postulation, bring one into being in the universe of physical things such that an average observer can see it also, then we're really talking about horsepower!! But under the right conditions, it is possible for particularly close, intimate or very sensitive people to see your mental creations. Just not usual and certainly not predictably.

A