The heart means love. A good heart means love to the Lord and to the neighbor, while a hard or stony heart means the love of self or the world -- riches and things. When the psalmist asks the Lord to search his heart, he wants the Lord to see what it is that he loves. And we are what we love. What we eat may form our earthly body, but it's only temporary, what we love forms our spiritual body, beautiful or ugly as we have chosen. It's interesting to note that the first multi-cellular motion of our embryo is the pulsating that starts in a twist of blood vessel that is to become our heart, and the last motion of our natural body is our heartbeat. It is there for all of our natural life. And of course there is a beating heart in our spiritual bodies when we come to put them on.

The meaning of "to seek" in the Bible is pretty straightforward, but there is a bit of nuance. In most cases "seek" or "look for" is connected with truth and knowledge, where "desire" is connected with love and affection. This makes sense; the idea of "seeking" is connected to sight, and sight corresponds to our intellect and capacity to understand truth.

(Referenties: Apocalypse Explained 550; Apocalypse Revealed 429; Arcana Coelestia 4719, 6770, 6771, 7021, 8298)