An Occupied Space Cannot Contain Two Opposites
A beautiful example of the contents of the heart by the great scholar, Imām Ibn al-Qayyim al-Jawziyyah.
It is better for your mouths to be full of pus until it makes it go bad than to be full of poetry.
For a space to accept what is being put in it, it has to be emptied of the opposite. The same way this is relevant with individuals and objects, it is also true for beliefs and wills. So, if the heart is occupied with loving and believing in falsehood, then it has no space for loving and accepting the truth. Just like if a tongue is busy uttering what does not have benefit, then its owner will not be able to utter what has benefit for him except if he stops talking about what is useless. Also if the different body parts are busy doing what is not obedience then it is not possible to keep them occupied in doing what is obedience except if he (the owner) frees them of the opposite. In the same way, a heart full of loving other than Allāh, wanting him, longing to meet him, and enjoying his company cannot be occupied with love for Allāh, wanting Him, longing to meet Him except by emptying it (the heart) from attachment to anything other than Him (Allāh). And the tongue cannot move to remember Him nor can the body parts move to His service except if he (the owner) frees them from mentioning and servicing other than Him. So if the heart is full of occupation with creation and sciences that do not have benefit then there is no space for being occupied with Allāh and knowing His Names, Attributes, Laws. And the secret of that is that the Isghaa (listening in concentration) of the heart is like the Isghaa (listening in concentration) of the ears, so if he listens to other than the Ḥadīth (meaning speech , words, subject) of Allāh, then there will be no room to listen to or understand the Ḥadīth of Allāh. Just like if he is inclined in love towards other than Allāh then he would have no room to incline in love towards Allāh. If the heart utters other than the remembrance of Allāh then it will not have any space to utter His remembrance by using the tongue. And that is why we find in an authentic narration from the Prophet (ṣallallāhu ʿalayhi wa-sallam):
"It is better for your mouths to be full of pus until it makes it go bad than to be full of poetry." 
So he made it clear that a mouth can be full of poetry and therefore it also becomes full of doubts, fictions and unexisting measures. Also the sciences that have no benefit, jokes, comedies, stories and so on. If the heart is full of that, then when the truths of the Qurʾān, and the sciences that can make it experience its full function and happiness, come to him, it will not find any space for itself and no acceptance (from it), so it bypasses it (the heart) and moves to another place. If you give advice to a heart full of its opposite, it will not find a way in and (the heart) will not accept it. It does not enter inside but it only passes by briefly and not from the inside. That is why it was said:
"Clean your heart from other than Us , and you will find Us
For our Highness is compatible with every clean heart
And patience is a chest for the treasure of being associated with Us
He who makes the chest compatible wins its treasure."
 Narrated by Sa'ad Ibn Abī Waqqās in Ṣaḥīḥ Muslim, Kitābush Shi'ir (Book of poetry) (27/5610).
Translated by Naʾil Ibrāhīm