All the knowledge we have gain so far is through objective study, there is always an object to study and we as student examine it, make observations and finally we reach to a conclusion, thats how we make opinion about everything we came across. On the otherhand there is a subject (us) without subject can there be any object? Answer is pretty simple No, without subject there can be no object. In deep sleep there is no subject hence no objectivity exists. The study of subject itself is called self enquiry. When the subject is seek, such enquiry is called self enquiry. When you see an apple, ask who see’s the apple? You say apple “I” see the apple. Now ask who am I? The nature of such enquriy will lead to self, that’s why it is called self enquiry.