The mind always likes to feed on thoughts. Sometimes this happens even when sleeping. Not only the mind feeds on thoughts it also regurgitates and chews on them. Just like a cow that chews on old grass that has been eaten before (Figure 1).
Lets see how it works. Mind gets the basic information about the external world from our five senses. This is shown in the mind works model. All the information that arrives is finally channeled to the mind as thoughts (Figure 2). They are stored as memory. They become mind objects and and can be retrieved and fed to the mind again and again. This become a continuous loop of feeding of thoughts to the mind (figure 4). This is what was meant by regurgitation and chewing of them again. This is also referred to as mental proliferation. We will explore this further in the next step.
Figure 2: This figure shows how the information from the senses flow to the mind. Mind eventually receives all the information from the senses. It can then retrieve them making a continuous loop.
In the Figure 3 it is shown how the mind feeds on thoughts. The eye is taken as the sense organ and an object taken as the sense stimulus in the example. We hang on to these thoughts triggered by the object with craving and this is what we call clinging to sensuality. The information that flows to the mind as thoughts are called the taints. As long as there is craving and clinging, mind is not free from taints. This is the pattern seen in an ordinary person (untrained mind). The Arahants have no craving or clinging. They therefore are free from all taints. There are no spontaneous flow of thoughts triggered by craving and clinging after the object is seen. These thoughts stop after its purpose is done. There are no spontaneous flow of thoughts. This is what is meant when said "in reference to the seen, there will be only the seen" in Bahiya sutta. These thoughts therefore do not generate any kamma (positive or negative) as they are free from taints (please see the post on taints for further discussions).
Figure 3: This figure shows the flow of thoughts in a ordinary person looking at an object.
In Figure 4 it shows that the visual object is no longer present. However the memory of the object already stored can be retrieved as mental objects and fed back into to the mind. This is how we feed the mind about the past objects we have seen. This is a type of mental nutriment (mano-sancetanaa). This process is called mental proliferation and in this case it is about the past. This is what was meant when it was said that the "mind chews on the regurgitated thoughts." Just like "a cow chews on old grass it has eaten before."
Figure 4: This figure shows how memory about the object is retrieved by the mind (craving is not shown in this figure).
We now know that the mind has to be fed. It is always hungry by nature. At least it is better to feed the mind with healthy food and not with junk food. Not only we feed on our old food we also have this habit of feeding on food scarps that other people throw at us. Sometimes we can get mentally sick or depressed feeding on them. There is an interesting story in Akkosa sutta how Buddha refused to feed on another persons words of abuse. So if you decide to feed on mental food at least feed on healthy food. Healthy food can generate good kamma. These are thoughts of your generosity, virtue and practice of meditation. Buddha compared higher mental absorptions (The four Rupa Jhanas) experienced through meditations like the internal food and medicine to the mind. However the ultimate goal is to stop feeding on all thoughts. This is the mind of an Arahant. This is when the feeding frenzy finally stops.
If you want to explore more on this subject please listen to these talks on "mental food" by Thanissaro Bhikkhu: