What is the different between perfect and imperfect rights?

–According to Salmond, “a perfect right is one which corresponds to a perfect duty. A perfect duty is one which is not merely recognized by law but also enforced by law.” In all fully developed legal systems, there are rights and duties which, though recognized by law, are not perfect in nature. Those rights are called imperfect rights. Example of such an imperfect right is the claim barred by the lapse of time. In such a case, the limitation does not extinguish the right, but bars the remedy only. The claim is valid in other respects, but it cannot be enforced. Such a case may be considered as an exception to the rule ubi jus ibi remedium.