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[Lieben's purpose in writing was to save his young friend and former student from the embarrassment that would come from making the rash suggestion that it might be meaningful to think about the arrangement of atoms in space. Five years later the irascible Hermann Kolbe excoriated young J. H. van't Hoff for publishing a pamphlet that dared to speculate about the arrangement of atoms (he even attacked the man who translated it into German). The philosophical content of Lieben's letter is almost identical to Kolbe's, but the tone is quite different. Note that in the original manuscript the word true is inserted as an afterthought, since Lieben wanted to stress that he was not just talking about the place you put a symbol in a formula.]

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Reprinted by Paternò in Gazz. Chim. Ital., 43, 501 (1913) when he was about to give up its editorship.

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My dear Emmanuele,

Let me comment on the isomerism which you and Cannizzaro have pointed out between
To imagine such an isomerism is certainly not absurd, even with no difference among the 4 valences of carbon. However I don't believe I err in supposing that the same reason which has kept me from adopting it will also keep all the other chemists from doing so (in a silent consensus).

Perhaps you have not thought that in assuming such isomerism you have crossed the Rubicon which separates speculation, which is considered permissible, about the way in which atoms combine from speculation, which is less permissible, about the true positions of atoms in space.

In all previous speculation one has always been careful never to consider the relative positions of the atoms in space (either distance or surroundings). Such an idea does not even enter into Kekulé's speculations on isomerism in the aromatic series. Kekulé believes that C⁶H⁴.¹Cl.²Cl differs from C⁶H⁴.¹Cl.³Cl and from C⁶H⁴.¹Cl.⁴Cl because in the first case the two CCl are joined to one another, in the second separated by CH, in the third separated by CH.CH, and to say this one does not need to hypothesize about the surroundings of the atoms. Perhaps it could be that the two Cl atoms in the 1:2 arrangement are in fact more distant from one another than in the 1:4! This does not enter into Kekulé's hypothesis.

If you hypothesize on the relative positions of atoms in space you would also make possible 2 CH²Cl², even granting the symmetry of the 4 valences of carbon and the existence of a single CH³Cl, for example:

I repeat that I do not consider imagining such an isomerism to be absurd, but since we have no means of knowing the topographical position of the atoms (while we have many for knowing how they are combined) I consider it a bit dangerous for science. Shooting off into space in search of atoms one risks losing the ground under his feet! Best regards to Cannizzaro, Körner, and Amato. I will soon write to Compisi.

Your good friend

Ad. Lieben

*/ Contrary to your hypothesis.