According to Leibniz's principle of identity of indiscernibles, if two objects have the same properties, then they are identical. To know whether objects are real or not, it is necessary to verify that two real things have the same properties. In our research, we aim to show that the principle of identity of indiscernibles is a logical principle and refers to a metaphysical deduction. In our research, we find that to apply the principle proposed by Leibniz to real objects requires a prior study of the properties of two things, and to apply the principle, we need to be sure of the premise that the properties of two things are the same, and to do this, we need to identify the real properties of things and compare them to each other in terms of identities. However, since it is possible to measure the existence of other, ordinary properties of things, we have seen that deduction about things themselves always lies on ground on the borderline between metaphysics and logic.