People can only see what’s on the outside, but the LORD looks at our heart, He knows everything there is to know about us.
I doubt any Christian or human being for that matter, apart from Jesus Himself could ever say they have a “pure heart”, so Jesus is trying to tell us that without His help, this is impossible.
We need to allow the Spirit of Jesus, that is the Holy Spirit, to prevent the filth of this world (sinful things in the world or the sin in our own lives) from sticking to our minds and corrupting our hearts. The Holy Spirit is the One who can pressure wash us on the inside to keep our minds clean and our hearts pure. Then we will be able to see God, that is, Him at work in our own lives and in the lives of others.