Bhagavad Gita Verse 32, Chapter 18

adharmam dharmamiti yaa manyate tamasaavritaa |
sarvaarthaanvipareetaanshcha buddhihi saa paartha taamasee || 32 ||
 
That by which one understands adharma as dharma, and all things as completely contrary, that intellect is called taamasic.
 
adharmam : adharma
dharmam : dharma
iti : in this manner
yaa : that which
manyate : one understands
taamasaa : darkness
aavritaa : covered
sarvaarthaanvipareetaan : all things as completely contrary
cha : and
buddhihi : intellect
saa : that
paartha : O Paartha
taamasee : taamasic
 
Let's imagine that there is a house with three rooms, each having a 100 W light bulb. One room is cleaned and dusted daily, so the bulb light shines brightly. All objects in this room are seen crystal clear. The second room is cleaned once every month, so the light from the bulb is partially covered by dust. Some objects in the room are seen clearly, but some are fuzzy. The third room has not been cleaned for several years, so the bulb delivers hardly any light at all, since it has acquired a thick coating of dust and dirt on it. We can barely see any object in this room.
 
Similarly, our intellect, which is like a light bulb, gets covered by the dirt of selfish desires. A raajasic intellect is like the bulb in the second room, with partially obscured light. But the taamasic intellect is like the bulb in the third room. The level of selfish desires is so great that the intellect cannot shine through. Shri Krishna says that such a such a person behaves in a totally ignorant and illogical fashion, confusing what is right with what is wrong. Vipareeta, the word used to describe such an intellect, means topsy turvy, contrary, reverse.
 
Where did such a high degree of selfishness come from? It is nothing but a bundle of vaasanaas, impressions that have been gathered since birth, or even through several lifetimes. It starts with the taamasic jnyaanam, the knowledge or worldview, that presents one object, person or situation as the sole goal of attainment, to the exclusion of everything else. It says, "money is the sole aim of life, everything else is secondary". So the intellect responds : "let's rob someone to get this money", and in doing so, going against all logic, morality, ethics and civility. Each time such an action is committed, its strengthens the vaasanaa for stealing and harming people. Over time, a thick cloud of these harmful vaasanaas coat the intellect.