When we look at Matthew 13:11-23, we can clearly see that it talks about a spiritual seeing and hearing, about the things of God. In verse 15, it says that a person understands with their heart. I believe, that is talking about revelation from God. To understand the things of God with the heart, means to understand them with our born again spirit, which receives the revelation from the Holy Spirit, living in us.
In fact, if we have only a head knowledge of the Bible, we can mess things up, because the Bible, which is the Word of God, is a spiritual book, the very Word of God, and its author, is the Holy Spirit, who inspired people through the ages, to write the thoughts and words of God, on paper.
Verse 23 talks about understanding the Word. That is understanding by revelation in our spirit from the Holy Spirit. That is define telly not an understanding of the mind. It is about revelation.
Only that type of revelation, will produce the good fruit, verse 23 is talking about.
Let Us Study the Bible 