Basis: We all have qualia, internal subjective sensations. You can never know if you experience The same Qualia as someone else even if we had a perfect neuroscience.
Thought experiment: Imagine we created a machine called "The Drake Machine". this machine is crafted to help humans empathise more deeply with the experience of others. First we place the machine on to subject A and have him prick his finger on a cactus spine. The Drake Machine being crafted by a perfect neuroscience records the exact firing off of every single neuron in this persons brain. Next we put the machine onto subject B. At this point The Drake Machine will fire off the exact same neurons in subject Bs brain that originally were recorded in subject A. This esentially allows Subject B to experience a play back of the experience in the form of a hallucination. However Subject B would still experience different qualia. Because when subject B's neurons fire off as recieving pain he can still only feel pain as his neurons are capable of firing off, his usual pain, and not the pain of subject A. To continue the experiment when Subject B experiences this hallucination The Drake Machine again records this experience. we then place the machine on subject A. we have him replay the last recorded experience, he would not experience the pain that was felt by subject B. He still would only feel pain as it feels to him when his neurons fire off. To him he would only relive the original experience.