The following if from the Ofudesaki:

Kono suino doko ni aru ya to omouna yo

Mune to kuchi to ga suna to suino ya 3-11

This filter, where do you think it is.

The heart/soul and the mouth are the sand and filter 

This verse I believe is one of the most important verse in the Ofudesaki.  It tells us how we can purify our minds by ourselves.  But first let talk about the heart or soul that is mentioned in the Ofudesaki many times, that God tells us that we do not understand. Our physical bodies decays, but there is something in our bodies that continue, which I call the soul or is referred as the heart in the Ofudesaki. It is this soul that collects and stores our merits or demerits on our actions while we are physically alive; and it is the soul that eventually distributes justice from these demerits. We reap what we have sowed.  But it is also the soul by itself that carries our thoughts patterns (dusts) after our physical demise, and delivers it to our next abode that we borrow from God.  The soul carries both our fates and thought patterns, as they both have a close relationship to each other.

Now lets interpret the verse (3-11) above.  The heart/soul functions as sand, as it collects our demerits when we act on our evil thoughts. It acts like the sand in our waste management system in attracting waste and eventually disposing of it. The soul functions to collect our demerits. The soul eventually eliminates the demerits by distributing it back to us as suffering or inconveniences in our lives.  People call this karma or innen.

The mouth is the filter because we have the ability to speak what is in our mind, or suppress it.  The meaning of the mouth includes any actions that we take upon any inconveniences or suffering we may encounter.  We can either act or these inconveniences or suppress them.  Now let us see if we can see the relationship between the soul and the mouth in purifying (filtering) our minds.

The soul collects our demerits and distributes it back as suffering or inconveniences back to us. We can calm our minds and ponder our evil thoughts that brought upon these inconveniences, and not act upon them or we can act upon them. If we act upon our evil thoughts when inconveniences occur, we continue the cycle of suffering. But if we calm our minds and ponder, we can suppress our evil thoughts, and purify our minds.  

Some may say that karma or innen is suffering only; but the soul actually helps us purify our minds, if we use the mouth appropriately! It is this that God refers to in verse 1-1, saying we do not understand the heart/soul.  We can all have the joyous life if we understand the soul.