Wolff had risen up as an opponent of the so-called preformation theory, still widespread at that time, according to which the entire plant with all its different parts is already present in embryonic physical form in the seed, and simply grows out into space through physical enlargement. Such a mode of thought seemed inadmissible to Wolff, for it made use of an hypothesis 'resting on an extra-sensible conception, which was held to be thinkable, although it could never be demonstrated from the sense world. Wolff laid it down as a fundamental principle of all research that 'nothing may be assumed, admitted or asserted that has not been actually seen and cannot be made similarly visible to others'. Thus in Wolff we meet with a phenomenologist who in his way tried to oppose certain trends of contemporary biological thinking. As such, Wolff had made certain observations which caused him to ascribe to the plant features quite similar to those which Goethe had grasped under the conception of progressive and regressive metamorphosis. In this way Wolff had grown convinced that all plant organs are transformed leaves. True to his own principle, he had then turned to the microscope for his eyes to confirm what his mind had already recognized.