Firstly, the post title is derived from the letters written by one of the groundbreaking thinkers, who added a new paradigm to the world as seen today- Karl Marx.
I shall not be concentrating on him, or his letter as yet. But, would like to revolve around this radical sentence : Ruthless criticism of everything that exists.
One of the main objectives of our existence is to validate the existence of everything else by our own critical faculties; In pursuit of this validation is when many paradigm shifts have been unleashed on the world.
We might tend to accept, with many generations of like minded people having lived before us for centuries, the need to criticize everything ruthlessly might not be necessary; But, chances that the most obvious of the things as perceived by the humanity are the ones which need the most criticism. For, given the obviousness of the entity, the chances that it has bypassed the scrutiny of the critical faculties of many generations is very high. To quote a few scantly scrutinized percolations of obvious human wisdom are God, Religion,Traditions,the Social fabric, the educational, economic, and political systems.
It naturally is our responsibility to put such percolations of supposed human wisdom under the lens of scrutiny with the bettered tools of criticism. And the word ruthless criticism is to be taken positively, which implies, it must be carried out with no prejudices, biases and compromises. Only then, the society with all its components can sustain and perpetuate based on reason, with incessant validations.
Ruthlessly criticize everything that exists! Move on, only when convinced.