Although Kardec was not a scientist by profession, linked to a specific branch of the sciences, he was the holder of a “scientific spirit”, as it contained all the conditions indispensable to the profession: 1) Serenity towards mediumistic facts, neither denied nor affirmed in principle; 2) Self-control so as not to be enthusiastic about the first results; 3) Be careful when selecting communications; 4) Caution in declarations, to avoid hasty disclosures of facts not yet examined and proven and; 5) Humility to seek the truth before and above all.
Spiritism and Science are not antagonistic forces, but on the contrary, they complement each other. Science, properly speaking, has as its object the study of the laws of the material principle, while Spiritism has as its object the knowledge of the laws of the spiritual principle and, as this is one of the forces of nature that reacts on the material principle, knowledge of one is not complete without the knowledge of the other. In this sense, it is observed that Science without Spiritism cannot explain certain phenomena using only material laws. In the same vein, Spiritism without Science becomes empty of support and proof.
In the process of codifying Spiritism, Allan Kardec used the experimental method, similarly to that used by positive sciences. Such methodology was based on observation, comparison, systematic analysis and conclusion. There was no preconceived theory about the existence of spirits and other doctrinal principles, the conception of which occurred only after the observation and verification of the facts. In other words, it was not the observation of the facts that resulted in the confirmation of a theory of Spiritism, but the theory arose only after the verification of the facts, whose purpose was to explain them. Therefore, Spiritism is a science of observation, and not a product of man's imagination. 
In the exercise of the experimental method, he tried to trace the effects back to the causes through deduction and logical linking of the facts, only admitting an explanation as valid when it solved all the difficulties of the problem, having always proceeded in this way in his previous works. Although Kardec used the inductive method to weave the first foundations of the Spiritist Doctrine, observing different examples to draw a general conclusion, he also used the so-called deductive process, reasoning by which conclusions are drawn through given premises, a since scientific processes require a combination of both.
Aspect of fundamental importance with respect to the Doctrine codification method is what we call universal control of the teachings of the Spirits, characterized by two important aspects: The generality or universality of the teachings, which have universal validity, having God entrusted the Spirits with taking them from one corner to another of the planet, manifesting itself everywhere; the agreement of the teachings, since they used a large number of mediums who were strangers to each other and in various places, but all in agreement with the teachings.
Returning to the theme in Genesis, Kardec teaches that generality and agreement in teaching are the essential character of the Doctrine, the very condition of its existence, resulting in any principle that has not yet received the consecration of the control of generality cannot be considered an integral part of the same doctrine, always being an isolated opinion, and Spiritism cannot assume responsibility.
The spirits' concordant opinion, examined by logic, constitutes the strength of the Spiritist Doctrine, ensuring its perpetuity. For it to change, the universality of Spirits would need to change its opinion, saying the opposite, since it has its source in the teaching of Spirits and, for it to succumb, it would be necessary for Spirits to cease to exist. 
Thus, it is observed that Kardec understood in his heart the gravity of the phenomena he was willing to observe, and the way in which they could revolutionize ideas and beliefs. Because of this, he tried to act with great circumspection, being positivist and not idealistic, so as not to be contaminated by illusions.
KARDEC, Allan. A Gênese. 53 ed.Brasília: FEB, 2013. p. 22. KARDEC, Allan. A Gênese. 48. ed. Rio de Janeiro: FEB, 2005. p. 11.