When you give attention to a theme frequently it becomes an inclination due to that frequent pondering.
What one sees that one thinks about and that theme is explored as basis for happiness, equanimity or distress.
Forms can be based on the eye or they can be a memory or imaginary.
Having direct one's attention to an object one explores it and associated themes according to one's best knowledge of good and bad (discernment faculty).
This is where one needs to pay a lot of attention and not be careless in giving attention, as to condition the mind properly.
What one sees, that one thinks about. What one thinks about that one objectifies. What one objectifies one evaluates. Having directed one's thought and made an evaluation one breaks out into speech; therefore thoughts & evaluation are verbalized.
These six are classes of craving: craving for forms, craving for sounds, craving for smells, craving for tastes, craving for tactile sensations, craving for ideas. This is called craving.
"And which are the 18 craving-verbalizations dependent on what is internal? There being 'I am,' there comes to be 'I am here,' there comes to be 'I am like this' ... 'I am otherwise' ... 'I am bad' ... 'I am good' ... 'I might be' ... 'I might be here' ... 'I might be like this' ... 'I might be otherwise' ... 'May I be' ... 'May I be here' ... 'May I be like this' ... 'May I be otherwise' ... 'I will be' ... 'I will be here' ... 'I will be like this' ... 'I will be otherwise.' These are the 18 craving-verbalizations dependent on what is internal. "And which are the 18 craving-verbalizations dependent on what is external? There being 'I am because of this (or: by means of this),' there comes to be 'I am here because of this,' there comes to be 'I am like this because of this' ... 'I am otherwise because of this' ... 'I am bad because of this' ... 'I am good because of this' ... 'I might be because of this' ... 'I might be here because of this' ... 'I might be like this because of this' ... 'I might be otherwise because of this' ... 'May I be because of this' ... 'May I be here because of this' ... 'May I be like this because of this' ... 'May I be otherwise because of this' ... 'I will be because of this' ... 'I will be here because of this' ... 'I will be like this because of this' ... 'I will be otherwise because of this.' These are the 18 craving-verbalizations dependent on what is external.
"Thus there are 18 craving-verbalizations dependent on what is internal and 18 craving-verbalizations dependent on what is external. These are called the 36 craving-verbalizations. Thus, with 36 craving-verbalizations of this sort in the past, 36 in the future, and 36 in the present, there are 108 craving-verbalizations.
As long as discernment remains undeveloped there will be ignorance on to that extent and on account of that ignorance what one sees and thinks about will be explored as basis for happiness on account of wrong views, and on account of these wrong views there will come into being wrong thoughts and resolves.
Therefore as you can see the discernment is to be developed and it leads to development of the path as it is a condition for right thought and resolves.