Considering the relevance of thermodynamics to the scientific discipline of chemistry and the curriculum of the Western school system, the philosophical system of Mario Bunge, particularly his ontology and epistemology, is used herein to analyze the presentation of the first law of thermodynamics in 15 school and university textbooks. The ontological analysis shows that the concepts heat "q" and work "w" are categorized as processes, while the concepts of internal energy "U", pressure "P", volume "V" and temperature "T" are categorized as properties. The results reveal that 8 of the 15 textbooks incorrectly present work "w" as a property, while 7 textbooks incorrectly present heat "q" as a property. Furthermore, 3 textbooks present the concept of energy as a property and assign it a merely operational definition as the capacity to do work. The analysis also examined patterns of causality and the mechanism used to explain the connection between the variables of cause and effect in three thermodynamic systems. The results indicated that only 2 textbooks contain such a mechanism.