In the 17th century, Leibniz discovered the "Law of Identity of Indiscernibles". The law says that if some object X is identical to some object Y, then object X and object Y must have all the same traits. Philosophers thought that there may be two kinds of traits: "intrinsic" and "extrinsic". Intrinsic traits are the traits that make the object what it really is. Extrinsic traits are traits that are about its relationship with other things. So to solve the problem of change, some philosophers have argued that what is changing are the extrinsic traits, but the intrinsic traits remain the same. So for example, while your body has changed over the years, there may be intrinsic traits to you that have not changed and make you who you really are. This could be a soul, psychological consistency, or something else.