Right from the day you came know that 'you are' to this day you still know that 'you are'. All add-ons have come and gone. They are transient but the fundamental 'I am' has remained unchanged and is the only certainty. This 'I am' is impersonal, it's common to everybody and wordless, the moment you came know that 'you are' you did not know any words or language, which came later. Based on this non-verbal 'I am' you could later on say verbally 'I am' in whatever language you were taught. From this tiny, minuscule 'I am' further knowledge grew leaps and bounds to gigantic proportions. So all knowledge stems from the 'I am', it is absolutely fundamental, the base, the origin, the root of everything. You have to hold on to this 'I am' and let everything else go.