Matthew 12:35 compares the human heart to a treasury, storing what you see as valuable. You can only draw from this treasury what you stored there, so a good person brings forth good, while an evil unbeliever has only an evil capital in his unbelief, giving only evil interest.
In Matthew 12:35 Jesus compares the human heart to a treasury. Everyone stores up in it what he considers valuable. When something is needed, one draws from this treasury—but only that which has first been stored there can be taken out. As such a good person brings forth that which is good, while an evil person can only bring forth what is evil.1
Although the heart is the source of much of life, the emphasis here lies on the fact that the good person (the believer) has been entrusted with a good treasure from which he can draw good things, while the evil person (the unbeliever) possesses only an evil capital in his unbelief, which yields nothing but evil interest.2
35 The good person out of his good treasure brings forth good, and the evil person out of his evil treasure brings forth evil.