To know one has to experience for themselves, the proof cannot be measured by scientific instruments or logic, there are methods to attain knowledge of spirit, blast me all you want but that is what the yogis, saint,sages and mystics, true philosophers for thousands of years would tell you. You will probably call them all quacks too. If you don't want to use the method of finding out for yourself because you are lazy and cynical then you will never know and I can never convince you not that I am speaking from full knowledge on such matters. Plato himself would agree with me.