Nursing assembles knowledge from different theoretical sciences and applied areas, involving the biological, human and social. Production of this specific knowledge aims to contribute to improve the quality of care and obtain better levels of health for human beings. Assuming truth of knowledge is the basis that determines the understanding of how people relate to each other. It is worth questioning the adjectives connected with it and actions taken towards it, in contexts that can be real or not: what is the range of rationality of knowledge? What is the extent of the diversity of knowledge? Is it possible to trust this knowledge and its rationality? Overcoming antagonisms among perspectives previously considered opposite, excluding and incompatible, in their epistemological and ontological positions towards the nature of research and society, makes it obsolete to defend the descriptive positivism of numbers in contrast to the descriptive qualitative production of words. Nevertheless, forms of knowledge legitimacy need to be reconsidered, approaching factual truth (quantitative) to the truth of discourse (qualitative). Quantitative, positivist research aims to document objective facts of reality and is interested in the numerical expression of phenomena. This presumes the separation between observing subject and observed “object”, and supports the existence of an objective reality, subject to “immutable” laws. Qualitative research aims to reconstruct symbolical patterns, expressed in an intersubjective way; is interested in heterogeneity and relational logic, expressed in words and images. It accepts the role of ideology as well as subjectivity. In principle, qualitative and quantitative research are distinct paradigmatic forms of conceiving reality, of conceiving the determining axes of social tissue, methodological instruments and strategies, and ideological reflections about human problems. Nonetheless, exact sciences are not free of values since, in the act of knowing, there is interest in transforming. On the other hand, subjectivity alone does not manage to cover the context it is formed in or which it transforms. Besides that, the intangible nature of quality “falsely” suggests its imponderability. The mathematization of the world coincides with the appearance of the bourgeois society. The 20th century marks the strengthening of the theoretical and epistemological inquisitive bases, from the quantitative and statistical perspective. It tries to find laws independently of the social, cultural, historical and political contexts. The cause-effect relation is present in the design of dependent and independent variables, where knowledge of the phenomena is valid if it is countable, measurable or provable. Marxism caused an intense reflection about the basis of knowledge, but also an overvaluation of the social. Neo-positivism excludes objective naivete, recognizes the lack of radical subject/object maintenance and uses different methods in a complementary way. It considers science free from preconceived values, searching for explanation, prediction and control of phenomena, not only in numbers but also in words. The constructivist paradigm is opposed to positivism as it does not intend to predict, control or transform the natural world, but tries to reconstruct and understand the universe, from the mind of the people who form it. In applied fields like health, integration of these different perspectives has been more and more frequent, due to the complexity and level of knowledge regarding human and social problems. Thus, the use of mixed or multiple methods is a tendency in nursing.