As a man whose head is under water cannot inhale pure air, so a man whose thoughts are plunged into the cares of this world cannot absorb the sensation of the world to come.
St. Isaac the Syrian
I think this is largely true, and the Bible seems to support this, eg the seeds in the parable of the sower which were choked by the cares and worries of the world, or the command (Psalm 46) to "Be still and know that I am God". We are specifically told not to spend all our energy on mere earthly survival, but to seek first His kingdom and His righteousness, and these things shall be added unto us. It is like being tuned to the wrong channel, so that the message doesn't get through. But I want to modify the statement a little. as it stands, it is dangerously close to saying that our own steadfast concentration (pity help the one with ADD)can keep us in the presence of God, as if that were a work we could accomplish on our own. As if there was no sin of spiritual pride to catch us out, more deadly than any distraction of the flesh. As if part of our journey did not have to lie through the dark places where we learn our need of God by experiencing His seeming absence. I am thankful that God's reality in my life is not only dependent on my own efforts, but on the grace that breaks through, that interrupts my downward spiral and calls me back to His embrace.
Of course, as the mysterious St Isaac must have known, to keep your head under water for too long is actually to die ...