Mao Tsetung (1977), Five Essays on Philosophy, Peking: Foreign Languages Press.

On praxis:

Activity in production is fundamental to Marxist thinking and the determinant of all other activities. Therefore knowledge too arises from the activity of material production (now cast as immaterial of course). Human knowledge thus is developed out of relations of production and other social practices, bound together and expressing social class distinctions in all political and cultural activities. Mao Tsetung verifies this centrality of practice bound to dialectical laws: 
'The dialectical-materialist theory of knowledge places practice in the primary position, holding that human knowledge can in no way be separated from practice and repudiating all the erroneous theories which deny the importance of practice or separate knowledge from practice. Thus Lenin said, "Practice is higher than (theoretical) knowledge, for it has not only the dignity of universality, but also of immediate actuality."' (1977: 4; quoting Lenin's notes on 'Subjective Logic or the Doctrine of the Notion')
This is a description of praxis.

Through this dialectical and materialist movement of cognition, perceptual knowledge is deepened to become logical or rational knowledge unified through practice: 'Our practice proves that what is perceived cannot be at once comprehended and that only what is comprehended can be more deeply conceived. Perception only solves the problem of phenomena; theory alone can solve the problem of essence.' (Mao, 1977: 6-7). To Mao, these ideas are verified through that fact that revolutionary theory arises from the practice of revolution itself - knowledge from direct experience. To imagine that rational knowledge can arise in itself without practice is 'idealism' or that the perceptual stage is reliable knowledge in itself is 'empiricism' and both are founded on false principles and un-dialectical (although note that Hegel's use of dialectics can be described as idealist, Marx updated this by relating this to practice). As dialectical thinking, deeper knowledge or a theoretical understanding is not enough in itself but is only useful in as much as it must be applied through action. Thus theory is continually developed and tested in the practice of production. It begins with practice and returns to practice. 

Importantly this dynamical process does not stop as the contradictions remain. Mao says: 'The movement of change in the world of objective reality is never-ending and so is man's [sic] cognition of truth through practice. [...] Our conclusion is the concrete, historical unity of the subjective and the objective, of theory and practice, of knowing and doing, and we are opposed to all erroneous ideologies, whether "Left" or Right, which depart from concrete history.' (1977: 19) 
This is the dialectical-materialist theory of knowing and doing (unwittingly echoed in the early stages of the national curriculum in art), wherein practice leads to theory which leads to practice and so on. With each cycle a deeper understanding of the objective and subjective world is achieved: 'Discover the truth through practice, and again through practice verify and develop the truth.' (Mao, 1977: 20) 

On contradiction:

Materialist dialectics follows the law of contradiction in things, that lie at their very essence or the 'kernel' of the object according to Lenin (Mao, 1977: 23; citing Lenin's 'On the Question of Dialectics'). This view of the world stands in contrast to the metaphysical or evolutionist outlook that regards things, forms and species as isolated from one another and immutable. For instance capitalist exploitation can, in this way, be explained as something inherent to human nature, or social development simply explained by environmental factors. At the time of writing in 1937, Mao sees this as the tendency in Europe expressed as 'mechanical materialism' in pre-industrial times and as 'vulgar evolutionism' during the industrial age. The materialist-dialectical outlook, on the other hand, examines things both internally and in relation to other things: in other words, 'the development of things should be seen as their internal and necessary self-movement, while each thing in its movement is interrelated with and interacts on the things around it. The fundamental cause of the development of a thing is not external but internal; it lies in its contradictoriness within the thing.' (1977: 26). 
The internal contradictions provide the basis of change and the external factors the conditions for change. However, external causes become operative through internal causes (Mao, 1977: 28). In society, the internal contradictions are expressed between the productive forces and the relations of production, and provide the catalyst for the 'supersession' of the old by the new order. Importantly, as previously stressed, this new form also expresses new contradictions in infinite progress.

Contradiction exerts universal rules, as it exists in the development of all things in mind, nature and society: 'Motion itself is contradiction' (1977: 31; Mao citing Engels from the essay 'Dialectics. Quantity and Quality'). For Engels and Mao alike, without contradiction nothing would exist at all, let alone be able to develop or change. It is present in all things however simple or complex they are, from beginning to end. As there is nothing but this determining contradiction of matter in motion, Mao explains that through this different forms can be identified. There is a certain 'particularity of contradiction' or essence that can be observed from one form of motion to another, enabling the observer to distinguish between things. Every thing has its own particular essence or contradiction. His examples of this include: 'positive and negative numbers in mathematics; action and reaction in mechanics; positive and negative electricity in physics; dissociation and combination in chemistry; forces of production and relations of production, classes and class struggle , in social science; offence and defence in military science; idealism and materialism, the metaphysical outlook and the dialectical outlook, in philosophy; and so on.' (1977: 36). Interestingly, the examples end recursively, with the principle itself proving itself as its own particular essence and contradiction. 

The earlier dialectical movement of knowledge from practice to theory back to practice is further explained through contradiction as the movement from the knowledge of the particular to the knowledge of things in general. Thus the essence of things is further understood through general understanding, and continues the dialectical cycle of better understanding (described elsewhere as the move from ignorance to knowledge, or scanty knowledge to substantial knowledge, from blindness to mastery of method, Mao, 1977: 58). It follows that general and abstract truths are only useful if they are applied again back in the world to concrete things. This is the tendency of much academic theoretical work of course - kept at the level of abstraction, without application to concrete conditions - that Mao would call dogmatism: 'our dogmatists are lazy-bones' (1977: 37) like many academics. If performed correctly by 'concrete analysis' (to paraphrase Lenin), each cycle makes knowledge more profound.

In this way, contradictions are not resolved but perhaps temporarily or partially mitigated (1977: 43). Hence the process is marked by stages and there is no limit to further development: 'what is universal in one context becomes universal in another. Conversely, what is particular in one context becomes universal in another.' (1977: 48). 
Antagonism is an important form of contradiction, and the struggle of opposites for revolutionary theory. For instance, contradiction between classes must develop to the point of open antagonism for revolution to arise. By analogy, Mao explains that a bomb contains contradictions but: 'the explosion takes place only when a new condition, ignition, is present.' (1977: 69) Under capitalism or any exploitative condition, antagonism is thus a wishful temporary state whereas contradiction remains. In Mao's thinking, socialist societies do not contain antagonism but instead contradictions are temporarily resolved by the system itself in the process of continual development. Contradictions remain.