Consider these three definitions from the LDOCE:
mind – your thoughts or your ability to think, feel, and imagine things mental.
self – the type of person you are, your character, your typical behaviour etc.
soul – the part of a person that is not physical, and that contains their character, thoughts, and feelings. Many people believe that a person’s soul continues to exist after they have died.
Often the mind, self and soul are synonymous, but as the definitions show they are not used in the same way. There are no true or perfect synonyms.
While the definition of the mind does not mention character, the definition of self does not mention thoughts. The former is about ability; the latter about quality.
In the definition of the soul both ability and quality are brought together. It also contains or emphasises two further qualities – that of non-physical and (sometimes the belief in) its continuation beyond physical death.
I doubt anyone thinks that thoughts and feeling continues to exist in the mind after his or her death, or that one can be described as being a type of person with a certain character or behaviour after his or her death. It is with the soul only that we continue to think of someone’s continued existence beyond their physical one.
But what is this ‘part of the person that is not physical’ that no one has seen and everyone infers to exist? By what evidence does one have to make this inference? I can infer thought from physical reaction, and character from physical attributes. But one cannot infer the existence of a soul from death, apart from the cessation of thought, feeling and characteristics. What can only be inferred is that death is the cessation of these. Souls, then, are the thoughts and remembrances of the characteristics of those who are living, or have since passed.