So then they that are in the flesh cannot please God. – Romans 8:8

The unregenerate world of sinners despises the Holy Ghost, “because it seeth Him not.” Yes, I believe this is the great secret why many laugh at the idea of the existence of the Holy Ghost-because they see Him not. You tell the worldling, “I have the Holy Ghost within me.” He says, “I cannot see it.” He wants it to be something tangible-a thing he can recognize with his senses. Have you ever heard the argument used by a good old Christian against an infidel doctor? The doctor said there was no soul, and asked, “Did you ever see a soul?” “No,” said the Christian. “Did you ever hear a soul?” “No.” “Did you ever smell a soul?” “No.” “Did you ever taste a soul?” “No.” “Did you ever feel a soul?” “Yes,” said the man-“I feel I have one within me.” “Well,” said the doctor, “there are four senses against one; you only have one on your side.” “Very well,” said the Christian, “Did you ever see a pain?” “No.” “Did you ever hear a pain?” “No.” “Did you ever smell a pain?” “No.” “Did you ever taste a pain?” “No.” “Did you ever feel a pain?” “Yes.” “And that is quite enough, I suppose, to prove there is a pain?” “Yes.” So the worldling says there is no Holy Ghost, because he cannot see Him. Well, but we feel Him. You say that is fanaticism, and that we never felt Him. Suppose you tell me that honey is bitter, I reply, “No, I am sure you cannot have tasted it; taste it and try.” So with the Holy Ghost; if you did but feel His influence, you would no longer say there is no Holy Spirit, because you cannot see Him. Are there not many things, even in nature, which we cannot see? Did you ever see the wind? No; but ye know there is wind, when you behold the hurricane tossing the waves about, and rending down the habitations of men; or when, in the soft evening zephyr, it kisses the flowers, and maketh dew-drops hang in pearly coronets around the rose. Did ye ever see electricity? No; but ye know there is such a thing, for it travels along the wires for thousands of miles, and carries our messages; though you cannot see the thing itself, you know there is such a thing. So you must believe there is a Holy Ghost working in us, both to will and to do, even though He is beyond our senses. ~ C.H. Spurgeon

http://www.blueletterbible.org/Comm/spurgeon_charles/sermons/0004.cfm