Dialectics for Sartre is the very relationship that BEING-FOR-SELF establishes with BEING-IN-SELF. Material objects have no conscience, are inert to facts, rest in the fullness of their contingency. Man, on the other hand, the Being-for-itself who is also a Being-in-itself (body), has a conscience and this establishes relations of negation with objects, issuing judgments and observing their contradictions.
This movement, coming from Para-si, is the dialectic and Sartre just wants to argue in favor of this in detriment of the supremacy, until then established, of the Analytical Reason, in which it was believed that scientists could observe and describe with complete impartiality and objectivity the phenomenal world.
Sartre points out on this issue, showing, due to his method, that the relationship always takes place at the level of subjectivity. For him, it is impossible to achieve any objectivity without this not being partially subjective (having gone through subjectivity). There are no truths other than those attested by a conscience in a certain time and place.
Having established the dialectic in this way, Sartre's critique also falls on Marxism, which considers man's conscience determined by the historical-dialectical materialism. If for Sartre the conscience is free to make decisions and be responsible for them, he would never admit that history was or was part of a process material, that is, I would not conceive that the dialectic of history was determined by the material conditions of existence, but rather by the consciences of men and only of the men. History is characteristic of man, as only Being-for-itself is totalizing or has totalization-in-progress. The beings in themselves are already full.
What actually happened, therefore, says Sartre, is that the analytic method (Analytic Reason) was wrongly applied to explain a dialectical process (Dialectical Reason). The dialectic does not apply to nature. Being-in-itself or matter has no temporality (past, future); does not see its contradictions and therefore has no history. This is characteristic of men who, as they constitute themselves as a totalization-in-progress, in the nature of being, etc., always creates projects to achieve their goals, mirroring the past to build the future. However, their projects are never fully achieved, generating anguish.
By João Francisco P. Cabral
Brazil School Collaborator
Graduated in Philosophy from the Federal University of Uberlândia - UFU
Master's student in Philosophy at the State University of Campinas - UNICAMP
Source: Brazil School - https://brasilescola.uol.com.br/filosofia/consciencia-dialetica-historia-sartre.htm