I Corinthians 2:13

Which things also we speak, not in the words which man's wisdom teacheth, but which the Holy Ghost teacheth; comparing spiritual things with spiritual (KJV)

Then: Paul is swimming on the surface of deep water. He's trying to explain the spiritual in the physical realm and have it move into the spiritual realm of fellow believers. Tough. So the Holy Spirit speaks to Paul's inner man and teaches him ways to live and what to do to tell others the good news. Paul has to recognize the difference between his own thoughts and those of the Spirit. Then he uses God's words and ideas to talk to the Corinthians. He's also very careful to compare that which is spiritual to things which are spiritual, not physical. Henry elaborates that it is a waste of time to try to validate revelations of the spirit by looking at the things of science or art or anything of the physical world. Spiritual wisdom is revealed by comparing the side-by-side prophecies of the Old Testament with the work of Christ on earth and with the teachings given to the saints, apostles, or disciples.

Now: Spiritual maturity evolves with the faithful practice of prayer or conversations with the Spirit. Learning to discern the words of the Lord and separate them from personal thoughts takes a lot of time. Paul's guideline here is to be sure you compare things spiritual with things spiritual, i.e., the written words in the Bible, words given to other faithful Christians, and the past spiritual events of your own individual experience. If you thought the Lord told you to help someone, you did it, and God received glory; then later you will be able to recognize that same prompting of the Spirit as being valid, not personal or satanic.

Extra sources: Commentary on the Whole Bible Volume VI (Acts to Revelation) by Matthew Henry, online at http://www.ccel.org/ccel/henry/mhc6.iCor.iii.html