All these evils come from within and they defile. Defilement (also referred to as being rendered unclean) is a great sin among the biblical Hebrew people. As such, a great care was taken to ensure that the purity of both the body and soul was maintained (except those cases that could not be avoided such as defilement by child birth or menstruation). And in case defilement did take place, there were measures taken to restore purity, for defilement rendered one unworthy to be in the presence of God and the community. There were several ways through which one could get him/herself defiled: consuming unclean food, coming into contact with unclean things/places, as well as some instances of sexual intercourse. However, with time, many things/places/instances became occasions for defilement. Even failure to follow proper hygiene (as we saw in the Gospel account that we read yesterday) became occasions for defilement.
Treated as such, defilement becomes restricted to external purity and less attention is paid to internal purity. This is the notion that Jesus wants to correct. More attention is paid to the external cleanness at the expense of the purity of the heart. The human person in his/her entirety as the temple of God must be worthy of God's presence. While this temple of God can be defiled externally, Jesus reminds us of the fact that the great source of impurity is the human heart. It is from the human heart that the evils that can destroy the human person arises. It is our thoughts and words that defile God's temple, not failure to follow proper hygiene.