Khan writes that since spiritual development is to do with the mind, spiritual progress is just another name for intellectual progress or intellectual development. According to Khan, intellectual development is the main target of man. 
Khan says that the process of spiritual or intellectual development in fact begins with the deconditioning of this conditioned mind or the purification of the mind – Tazkia as mentioned in the Quran. The more one de-conditions the mind, the more one will gain in spiritual or intellectual development. Spiritual or Intellectual progress is just not possible without taking such a step, argues Khan. He further explains: 
The truth is that everyone is born spiritual. Everyone is Mr. Nature or Mr. Spiritual by birth. But, after birth, he lives in a society, which continues to condition him. Spirituality makes it possible to erase this conditioning and allows the individual to revert to the natural state in which he was born. Spiritual science is, in effect, another name for the deconditioning of the mind. There can be no spirituality without this.
In this respect, the human mind can be likened to an onion. In the centre of the onion, initially, there is a small kernel or the natural state of the mind. Then layers start forming, one upon the other over this inner kernel. This process continues until the inner kernel is totally covered with external layers. Now, apparently, layers alone are visible. The kernel is completely covered and remains invisible. To find the kernel of the onion, all the external layers have to be removed. Similarly, all the conditioning has to be removed in order to restore man to his pristine nature. This process of de-conditioning is similar to the removal of the onion’s layers. The whole concept of spiritual science is highlighted by this example.
Khan writes that once a noted scientist said that only a prepared mind can make a scientific discovery. The same can be said of spirituality. Here also a prepared soul alone can benefit from it. Therefore, first of all tazkia, purification or deconditioning of the soul, has to be done through contemplation or tafakkur and tadabbur. Tazkia aims at clearing one’s mind of all kinds of negative sentiments, otherwise contemplation will not work. There is a very interesting story, which illustrates this point.
Khan quotes a story of Gautam Buddha and the milkman.
A milkman came to Gautam Buddha and asked him to give him Gyan, i.e. the realisation of truth. Gautam Buddha asked:
“Would you like the milk to be put in an unclean container?”
The milkman said:
“No. First of all we must clean the container and only then can we put the milk in it.”
Gautam Buddha then said:
“The same is true of realisation. Realisation can be put only in a clean container. First of all, you have to cleanse your heart and mind of all kinds of moral pollution. Only after this cleaning process will you be able to receive the truth.”