It would be near impossible to articulate it. If one was asked to provide the skeleton and flesh it out, he would not be able to. The prospectus of his desires would be vague and ambiguous.
But when he ultimately sets his eyes on it, he would know instantaneously that is what he has always sought after. It would materialize as a sudden realisation. He would know that is what he wants and would perform everything in his power to attain it.
The problem is however, how often do desires and reality meld?