Suppose that one's dinner were served-bread, salad, meat, and dessert–in one homogeneous slab.
Nutritionally, of course, nothing would be absent.
But anyone sitting down to such a meal would hardly give it his complete attention.
It is certainly healthy, he might conclude, and-worth his while to eat; he chews and swallows.
But at best, only a reluctant fraction of his consciousness is actively involved.
Something similar happens on occasion when a person puts on tefillin and prays with reduced concentration and feeling.
He knows that these are matters that he must attend to and that his soul will benefit.
Yet, to a significant degree, his heart–the full capacity of his psyche–is not in it.
For the soul's attitude to the service to God to be optimal, there must be a full complement of awareness and feeling.
If even one layer of the soul's infrastructure is lacking, its service is deficient and the aftermath can lead to disarray and crisis.
—Rabbi Adin Steinsaltz
From Learning from the Tanya by Rabbi Adin Steinsaltz