If external objects did not exist, then how is it that one state of consciousness is not limited to just one person but experienced by many alike, so that a number of people see the same thing at a given place and time? This is one of the serious objections raised by realists against the Yogacara idealistic doctrine, mentioned by Vasubandhu in his Vimsatika. According to Prajñakaragupta, however, irrespective of whether external objects exist or not, one has to clear up the basic question whether we can say that others see the same thing as we see.
Sautrantikas argue: When others have symptoms such as the bristling of the hair of the body, we, seeing them, infer that others experience emotions such as joy and hence that others also see the same thing as we see. In order to reject this view of the Sautrantikas, Prajñakaragupta puts them into a dilemma. If they argue that others who are seen to have the bristling of the hair of the body are known to see the same thing as we see, then they have to accept the undesirable consequence that the joy experienced by others is not distinguished from that experienced by us. If, on the other hand, they want to avoid this consequence ensuing, then they have to accept that the thing others see and which gives them joy is different from the thing we see and brings us joy, that is, that others cannot see the same thing as we see.
According to Prajñakaragupta, in addition, any verbal act of others also cannot serve as evidence to show that others see the same thing as we see. For, we cannot perceive other minds. We cannot be perceptually aware that others see the same thing as we see, which is illustrated by the fact that two persons, who are afflicted by eye-disease (taimirika), cannot help each other. For Prajñakaragupta, even if two persons can speak of the existence of the external object, it is as if the two persons who are afflicted by eye-disease speak of the double moon.
Thus Prajñakaragupta's answer to the basic question mentioned above is: We cannot establish that others see the same thing as we see.