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. The microscope gave him the confirmation he expected by showing that all the different organs of the plant develop out of identical embryonic beginnings. In his absolute reliance on physical observation, however, he tried to go further than this and to detect in this way the reason why the plant does not always bring forth the same organ. He saw that the vegetative strength in the plant diminishes in proportion as its organism enters upon its later stages. He therefore attributed the differentiated evolution of plant organs from identical beginnings to an ever weaker process of development in them. Despite his joy in Wolff as someone who in his own fashion had arrived at certain truths which he himself had also discovered, and despite his agreement with Wolff's phenomenalistic principle, Goethe could in no way accept his explanation of why metamorphosis took place in plants. He said: 'In plant metamorphosis Wolff saw how the same organ continuously draws together, makes itself smaller; he did not see that this contraction alternates with an expansion. He saw that the organ diminishes in volume, but not that at the same time it ennobles itself, and so, against reason, he attributed decline to the path towards perfection.' What was it, then, which had prevented Wolff from seeing things aright? 'However admirable may be Wolff's method, through which he has achieved so much, the excellent man never thought that there may be a difference between seeing and seeing, that the eyes of the spirit have to work in perpetual living connection with those of the body, for one otherwise risks seeing and yet seeing past a thing (zu sehen und doch vorbeizusehen).'