“For out of the heart come forth evil thoughts, murders, adulteries, fornications, thefts, false witness, railings.”
Here ‘evil thoughts' is probably a summary of what is then given in detail. Thus the idea here is that evil thoughts come from the ‘heart' (that is, from the mind and will and inner being of a man). And that these evil thoughts then reveal themselves in such behaviour as murder, adultery, all sexually irresponsible behaviour, theft, and false witness. Note that here in Matthew these follow the order of the second section of the ten commandments, and much of what is in the Sermon on the Mount in chapter 5. They are then followed by ‘railings' (‘blasphemia' - injurious speech whether of God or men) which replaces ‘covetousness', but this may contain within it the idea that men do in fact rail against God and man because they do not get what they want. Thus their covetousness is revealed by what comes from their mouths. All this includes the idea in James that the tongue can be ‘a little member --- set on fire by Hell' (James 3:5-6) because of the harm that it can do. Notice also that adultery has been expanded to include all irresponsible sexual behaviour. Men murder, and hate, and destroy each other, and as they do so their tongues will reveal it in various ways. And they behave sexually irresponsibly, and steal, and cheat, and cannot be trusted, and belabour others and thereby again reveal themselves for what they are. And all of them will in one way or another result in words that come from the mouth. So it is not the world that contaminates them. It is they who contaminate the world.
Thus it is the evil thoughts within a person, which result in evil actions and in evil words, which are the true measure of uncleanness.