Karl Marx's concept of 'alienation' (Entfremdung) is indeed central to his critique of capitalism, positing it not as an accidental byproduct but as an inherent and essential feature of the capitalist mode of production. For Marx, alienation describes the estrangement of individuals from fundamental aspects of their human nature, their work, and their social relations under capitalism.

Marx identified four key forms of alienation:

1. Alienation from the product of labor: In capitalism, workers do not own the products they create. These products become commodities owned by the capitalist, sold for profit, and often used against the worker (e.g., as tools of further exploitation). The more the worker produces, the poorer they become, and the more powerful the capital that controls them.

2. Alienation from the act of labor: Work itself becomes external to the worker, a means to an end (survival) rather than an end in itself. It is forced, monotonous, and lacks creativity or self-fulfillment. The worker feels like a cog in a machine, not a conscious, creative being.

3. Alienation from species-being (human essence): Marx believed that humans are fundamentally creative, social beings who find fulfillment in purposeful, conscious activity that transforms nature. Capitalism, by reducing labor to a mere means of subsistence and stripping it of its creative potential, alienates individuals from this essential human capacity.

4. Alienation from other human beings: The competitive nature of capitalism, the division of labor, and the commodification of relationships lead to estrangement between individuals. Workers compete against each other, and the relationship between capitalist and worker is inherently antagonistic, based on exploitation rather than cooperation.

These forms of alienation are not incidental; they are direct consequences of private ownership of the means of production, the wage labor system, and the relentless pursuit of profit. For Marx, alienation is the 'reality' of capitalism because it systematically dehumanizes individuals, distorts their relationships, and prevents them from realizing their full potential, making it a fundamental flaw that necessitates a revolutionary transformation to communism.