Nowadays it is becoming common to claim that there is no such thing as truth; what would exist are merely interpretations, particular points of view (knowledge), all equally valid. And that any attempt at knowing the truth would be nothing but sheer arrogance and pretentiousness of those naively seeking to cage the complexity of existence within limits authoritatively imposed by whatever deterministic approach.
This view exerts an undeniable fascination over common sense and over many of the intellectuals who dominate the academic world, since it seems to give a great feeling of freedom, where the imposing absolute truths of the past are replaced by interpretations customized at the whim of individual preferences and even of momentary conveniences. This approach has found a large membership, even among militant sectors from social movements which see in a denial of the objective truth the assertion of their own agendas.
However, this relativism which, at first, seems to come to the aid of those who seek to build a society free from all forms of oppression and exploitation, is nothing but an illusion that relies on a shallow analysis of reality. It seeks to conceal the fact that this ideology of absolute relativism does not represent more than a return to the old ideologies of the past and is in service of the interests of the elites who encourage and deepen not only sexism, racism, homophobia and ethnic and religious intolerance, as well as the exploitation of workers and poor people in our society.
After all, although in the relativistic discourse of the postmodernists ideologues “all truths are equally valid”, in everyday social practice what we see is the imposition of a single truth, that of those who wield power. Of what use to us is the supposed “freedom” to choose from a multitude of equally valid approaches and points of view to analyze reality, when in practice we go on under the yoke of the capitalist logic that keeps the God-Market’s interests above any social or human interest?
The relationship between sectors of the left and the relativistic ideology that came to be called “postmodernism” or, more appropriately, post-structuralism, is a reflection of one of the worst tragedies in the development of the human thought, a tragedy fostered by the combined forces of the two greatest propaganda machines that humanity has been capable of producing: the powerful and pervasive capitalist mass media and the monstrous propaganda apparatus of the Soviet bureaucracy. For different reasons, each driven by their own interests, they buried the conquest that is dialectical materialism under thick layers of lies and distortions.
In the late 20th and early 21st century, large sectors oppressed by the capitalist hegemony and disillusioned with both the Enlightenment rationalism of the 18th century and with what common sense considers to be the Marxist proposal that emerged in the 19th century turned to the irrationalism and idealism which characterize the “postmodern” (or post-structuralism) ideology, searching for theoretical alternatives which could back the resistance actions that they were, and are, driven to perform by the advance of oppression and exploitation which has accompanied capitalist decadence.
Such a relationship was only possible because these oppressed sectors, betrayed by both the liberal bourgeoisie and the Stalinist bureaucracy, saw in the “postmodern” proposal something which, to them, seemed consistent with their just rejection of Enlightenment rationalism and pseudo-Marxist determinism, since the historical experience showed both the Enlightenment’s inability to implement its abstract promise of liberty, equality and fraternity, and the failure of the so-called “real socialism”, which was no more than a distortion of dialectical materialism engendered and disseminated by the Sino-Soviet bureaucracy throughout the 20th century, where dialectics was replaced by dogmatism.
On idealistic approaches, including the “post-modern” one…
From the point of view of the idealist philosophy, the basis of all phenomena can’t be found in matter, but in divine will, in universal reason, in the absolute idea or some other form of abstraction that is away from the concrete reality. Thus, the idealistic interpretation of the world offers us an idealized image of reality that denies the existence of a concrete reality or the possibility that we may be actually capable of understanding it through practical experience.
Either through the objective idealism of Plato (428 BC – 347a.C.) and Hegel (1770-1831), which recognize the existence of truth but postulate that it could only be achieved through deep metaphysical intellectual exercise; through the subjective idealism, released by the Irish Bishop George Berkeley (1685-1753), who simply denies the existence of any objective truth other than God and argues that things exist only insofar as they are perceived by the human or divine mind; or through the transcendental idealism of Immanuel Kant (1724-1804) which recognizes the objective existence of reality but says that we are not able to understand it fully and sets arbitrary limits for its understanding; or through any other idealistic strand, by denying or trying to shift from concrete reality to the realm of metaphysical abstraction the search for truth, the idealist philosophy turns out to express the need or convenience of avoiding direct confrontation with objective reality and attempts to justify the position of those who choose to believe only what pleases them or seems to offer them some comfort given the uncertainties of existence. Idealism also expresses a pretentious position which tries to impose our own particular points of view to reality, when the correct seems to be just the opposite.
That is, idealism, in its different forms, denies or seeks to establish arbitrary boundaries for the investigation of the objective reality. But, only those who might benefit from its negation would benefit from such an effort to deny or limit the human search for truth.
About the materialistic approaches, including the positivist one…
A genuine effort to understand reality must be a humbling exercise that implies we are willing to give up our own desires and preconceptions to embrace what material reality objectively proves to be correct, even if it clashes with our own preferences and idealizations. Obviously, such a selfless stance requires from us a hard, sustained effort and, sadly, is not capable of stopping us from making mistakes.
The materialistic philosophy committed itself always to find a physical explanation for phenomena, capable of overcoming the obscurantism and mysticism inherent in the different strands of idealism, seeking to transpose the limits arbitrarily set for the advancement of human knowledge. In the materialistic approach such arbitrary limits are replaced by the search for limits objectively imposed by concrete reality.
That is, materialism asserts that truth not only exists, it can be known through practical experience, in a way that the very concept of matter is no more than “the objective reality that exists independently of human consciousness and that is reflected by it “, and our surrounding world is nothing but matter in motion. For this approach, even the most abstract ideas and concepts are no more than the result of the activity of a material body (the human brain) and the reflection of material objects perceived and processed by this same material body.
Obviously, this materialistic approach, which underlies Enlightenment rationalism and positivistic determinism, could not be free from subjective interference, since the limits that we think are objectively imposed for the investigation of reality are, themselves, subject to subjective interpretation. As Lenin said in his classic analysis on the foundations of dialectical materialism, entitled The Three Sources and Three Component Parts of Marxism, expecting science, itself a social practice, to be impartial in a class society, which is torn between opposing interests, would be as naive as expecting the capitalist elites to willingly decide to give up their own privileges.
Far from invalidating the materialistic approach inherent to true science, this realization only reinforces the need for a humble scientific attitude, conscious of its own limitations and fallibility and of the importance of its self-regulatory mechanisms and meta-analysis.
The fact that the materialistic approach was embraced by the revolutionary bourgeoisie in its struggle to overthrow the nobility and, later on, conveniently abandoned by the same bourgeois class when it came to power, gives us indications of the strength and revolutionary potential of this approach.
At the present time the materialist philosophy maintains its revolutionary content; giving us extremely important tools to uncover the hidden truth beneath the idealistic fantasies fueled by the decaying bourgeois elite. Revolutionaries have no reason to fear the truth; history shows that it is our most powerful ally. As taught by Trotsky: “To show to the oppressed the truth is to open the way of revolution for them.”
A consequent materialistic approach, even (and above all) when it clashes with our desires and expectations, is essential for us to take the proper course in the face of major adversities. No wonder that in one of the most difficult and crucial moments of class struggle, Trotsky has recorded for posterity the following recommendations:
“To face reality squarely; not to seek the line of least resistance; to call things by their right names; to speak the truth to the masses, no matter how bitter it may be; not to fear obstacles; to be true in little things as in big ones; to base one’s program on the logic of the class struggle; to be bold when the hour for action arrives.” 
About dialectical materialism…
But if on one hand, in relation to the objective character of reality and of the possibility of knowing it, dialectical materialism is definitely in the field of materialistic philosophy as opposed to every form of idealism, on the other, it also seeks to move beyond the limitations of any earlier materialistic doctrines, incorporating dialectics to a cohesive critique of the determinism that had always been tied to the different materialistic strands.
The dialectical materialists recognize that reality is objective and independent of us: it exists on its own and is subject to laws that are unrelated to our will. But unlike the deterministic materialism, for dialectical materialists this does not mean that reality is static. Far from it, the objective character of reality shows its inherent fluidity and dynamism.
Reality is concrete and objective, but also is in constant movement and transformation. We can know truth through practical experience, but this truth is always partial and transient and need to be permanently susceptible to the criticism of practical experience.
And that is not all. The dialectical materialist critique of vulgar materialism is deeper still than the mere acknowledgment of the dynamic nature of reality, it also includes the need to overcome the gap between theory and practice that the ideologues of the various philosophical currents, both idealistic and materialistic, insist on keeping and deepening.
In their Theses on Feuerbach, in the Critique of Hegel’s Philosophy of Law, in The Holy Family and The German Ideology, Karl Marx (1818-1883) and Friedrich Engels (1820-1895) develop the concept of praxis by criticizing simultaneously both idealism and the materialistic approaches of the time. Deterministic materialism argued that human beings are determined by the circumstances (economic, social and natural), whereas idealism sees human beings as determined by ideas (thoughts, wishes and desires). Vulgar materialists claimed that we change because new circumstances cause us to change, while the idealists of different hues claim that we change because education, new ideas and new desires make us change.
The critique of Marx is that materialism, without having incorporated dialectics, “forgets that circumstances are changed by men” , while idealism forgets that“it is essential to educate the educator himself” . Thus, both deterministic materialism and idealism, in whatever variation, end up reproducing the structure of class society, i.e. the exploitation of man by man. At this point, Marx introduces the concept of revolutionary practice as “the coincidence of the changing of circumstances and of human activity” .
The revolutionary praxis is thus understood as a theoretical and practical activity in which the theory is constantly changed by practical experience, which in turn is also constantly modified by the deepening of theoretical concepts, so that neither the theory is crystallized as dogma nor the practice degenerates into an alienated process.
As stated by Marx himself:
“The question of whether objective truth can be attributed to human thinking is not a question of theory but is a practical question. Man must prove the truth — i.e. the reality and power, the this-sidedness of his thinking in practice. The dispute over the reality or non-reality of thinking that is isolated from practice is a purely scholastic question.” 
Or, as Lenin reiterates in his classic philosophical treatise:
“Thestandpoint of life, of practice, should be first and fundamental in the theory of knowledge. And it inevitably leads to materialism, brushing aside the endless fabrications of professorial scholasticism. Of course, we must not forget that the criterion of practice can never, in the nature of things, either confirm or refute any human ideacompletely. This criterion also is sufficiently “indefinite” not to allow human knowledge to become “absolute,” but at the same time it is sufficiently definite to wage a ruthless fight on all varieties of idealism and agnosticism. If what our practice confirms is the sole, ultimate and objective truth, then from this must follow the recognition that the only path to this truth is the path of science, which holds the materialist point of view […]The sole conclusion to be drawn from the opinion of the Marxists that Marx’s theory is an objective truth is that by following thepathof Marxist theory we shall draw closer and closer to objective truth (without ever exhausting it); but by followingany other pathwe shall arrive at nothing but confusion and lies.” 
This unbreakable link between theory and practice in order to transform reality was one of the greatest contributions of dialectical materialism to human thought. By proposing the end of the dichotomy between theory and practice, as well as the ending of the division between manual and intellectual work, the materialist dialectic approach unravels the mechanisms of alienation in class society and calls upon all interested in the real transformation of society not only to reflect on the reality that surrounds us, but also to take an active part in the struggles and pressing issues of our time.
At the present time, the negation of deterministic materialism is a fair and rightful sentiment that reflects the actual historical experience with the bourgeois revolutions and so-called socialist revolutions of the 20th century, which proved unable to free humanity from the shackles of oppression and exploitation. But to advance the knowledge and social practices existing in this beginning of the 21st century, we need to go beyond mere denial of past experiences: we must find a solution that points to a new direction, without returning to the idealism that did not prove capable of offering answers either.
The overcoming of this tragedy that makes, in a nearly hegemonic way, the human thinking of this beginning of the 21st century to turn back to obscurantism and pre-Renaissance irrationalism necessarily involves the rescue of an approach that simultaneously counteracts materialism to idealism and the dialectic to determinism, breaking the boundaries between the different fields of knowledge, as well as those allegedly existing between theory and practice. After all, as summarized Marx: “The philosophers have only interpreted the world, in various ways; the point is to change it.” 
- LENIN, V. I. Materialism and Empirio-criticism, https://www.marxists.org/archive/lenin/works/1908/mec/two6.htm
- TROTSKY, Leon. The Transitional Program.
- MARX, Karl. Theses on Feuerbach. https://www.marxists.org/archive/marx/works/1845/theses/theses.htm
- LENIN, V. I. Materialism and Empirio-criticism, Chapter 2, Part 6. https://www.marxists.org/archive/lenin/works/1908/mec/two6.htm
- MARX, Karl. Theses on Feuerbach. https://www.marxists.org/archive/marx/works/1845/theses/theses.htm