Why Florestan and Eusebius?
The Musical Times, Feb. 1967 (pp. 131-134)© The Estate of Eric Sams
And twenty more such names and men as these
Which never were, nor no man ever saw.
What’s in a name? (ibid.)
For Schumann, everything was in a name, and his own was legion. Most famous of all are Florestan and Eusebius, the active and the passive aspects of his own personality.
No very compelling reason for this notion, let alone those names, has ever been suggested. A useful device for the writing of music criticism, perhaps? A confession of schizophrenia? Well, Schumann's was certainly a dual nature (which may have influenced his choice of twos as pseudonyms);  but on the plainest evidence his duality was cycloid rather than schizoid.  In any event it is not just Schumann but human nature to have both introvert and extrovert character traits. That is why the device of related but contrasting characters is found in fiction - which is where Schumann found it. “In all his works Jean-Paul  projects his own personality, and each time it is in two contrasting characters", he notes.
It is of course easier to project one self (i.e. one's self) as two characters if one is oneself their author. So it is not altogether surprising that Schumann in his impressionable adolescence began a novel on that plan. But it seems odd that an idea which expressed a boy's imitative interest in literature should also express a man's original genius for music.
One inference is that two-in-one contrast is essentially a musical idea. Indeed, it has been said to be the essential idea of music: “patterns of motion and rest, of tension and release, ... fulfilment, excitation. . .'"  or, as one might say, F and E). Another inference is that for Schumann literature and music had a common symbolism. This too seems puzzling at first. But the answer is as simple as – indeed, is – ABC; music and letters.
Now, we have only to look at those letters with Schumann in mind to recall that Ambrosia and Beda were two of his names for his beloved Clara signifying contrasting aspects of her personality. Further, he is himself famous as David in his unceasing mental fight against Philistines, with the help of his two lieutenants Eusebius and Florestan signifying contrasting aspects of his personality. There is far-reaching evidence of his obsession with the idea of musical letters, and the symbolic union of names. It seems likely that Ambrosia, Beda, Clara, David, Eusebius, and so on, exemplify just such an idea in its initial stages.
If so, the egregious fancy of Eusebius and Florestan would have been born plain E and F. This would explain why those letters are often used as signatures of the critical articles, and even of the music - as in the Davidsbündlertänze op 6, where Einfach describes two pieces signed “E” and Frisch two signed “F”. It would also explain one or two extravagantly far-fetched phrases in the correspondence. 
On this view Schumann would have begun by asking what names, beginning with E and F, could be directly related to himself, to each other, and to Clara? With hindsight we can see clearly what idea it is that links the quintet A-E above. For details consult any handy menology or synaxarion. But if the answer is not easily forthcoming there is another approach. Is there any factual evidence that Schumann at the material time had any particular kind of name in mind?
Indeed there is. It happened that in 1829 Schumann found the perfect companion in Willibald Alexis, the author, and instead of going direct from Frankfurt to Heidelberg he seized this opportunity to accompany Alexis as far as Coblenz on his way to Paris.  It must indeed have been a magnetic personality that caused this prodigious deflection in Schumann's itinerary. Its owner had himself read law at a university - the very reason for which Schumann was on his way to Heidelberg; and he had also given up law for art - the very reason for which Schumann was to leave Heidelberg in the following year. So the meeting may have been momentous for music in more ways than one. But the talk would have been mainly about letters. Both men had recently met Heinrich Heine; both were passionately interested in writers and writing; and Schumann was passionately interested in names and disguises. Of course he would have asked his “perfect companion”, whose real name was Wilhelm Häring (1798-1871) to explain the pen-name “Willibald Alexis”.
Häring could claim to be an expert on pseudonyms, having made a successful début as a writer of historical fiction by the novel expedient of calling himself “Sir Walter Scott”. Perhaps it was a sense of ironic humour that later prompted him to write under a name compounded of two saints-whose feast-days both fall in July.
The idea of informal self-canonization might have had a similar appeal to the young Schumann, who was no saint. But in 1829 he had just met one; Clara Wieck, his piano teacher's ten-year-old daughter. She was already famous as a child prodigy. To Schumann she seems to have been as notable for her virtue as her virtuosity. Before long he was thinking of her - rather surprisingly - 'as a pilgrim thinks of an image un a distant altar'. The pilgrimage was to be a long one. But there is no reasonable doubt that Schumann had already set out on it, all cockle hat and staff, by 1830; and it certainly ended at the altar. Meanwhile much of his music, as well as the whole of his life, was dedicated to his Clara. From the first, his pet-names for her show his adoration. Thus “Cilia” (or “Zilia”) is a reference to St Cecilia, traditionally a keyboard artist and patron saint of music and musicians. Furthermore, she was already in a sense a saint in her own right. There are few more Christian names than that of Clara, anglice Clare, of Assisi, justly celebrated in particular un her feast-day, August 12.
So Schumann could best symbolize a communion of saints by finding two of his own, with feast-days in August, and names beginning with E and F.
Ladies first is a very proper rule. Otherwise Schumann might have gone down to posterity as Ethelwold and Faith, both on August 1. They may still figure in processions, but seem unsuitable for Carnaval. Following Clara, the first eligible saintly E was St Eusebius, on the 14th. There too, the next beginning with F, is St Florus, on the 18th. All three images nestle in the niche of the same week. The rebaptismal ceremony is complete.
Or perhaps not quite. St Clara and St Eusebius are still just one day apart in many million missals. Schumann freely drew attention to the conjunction, as if the holy calendar itself had been arranged to bless their union. A sceptic might think that the agency was more likely to be Schumann than divine. Yet the coincidence has generally been accepted, despite its wild implausibility, and the obvious explanation has been rejected. The reasons for this prejudice are clear enough now; and they would have been even clearer to Schumann at the time. The mystery-lover's act of symbolic union, even though purely nominal, is properly concealed. Eusebius was enough; Florus would go too far.
The problem then would be to find a pseudo-pseudonym related to Florus but still closely connected with Clara. He would have found it by 1831, when Florestan and Eusebius first emerge. And in that same year we find a diary entry that shows he need not have looked very far.
“Clara befiehlt schon wie eine Leonore”,  he notes; Clara already gives orders, like a Leonora. We can hear the tone of voice, teasing and affectionate. There could be no better symbol of unity with so prima-donna-ish a character than the name of her operatic husband, in a work which Schumann worshipped, which celebrates - and indeed is called Married Love.
Therefore Eusebius and (not Florus but) Florestan. Why the one was thought apt for inward, the other for outgoing, moods is another story. But it will be found, like most of Schumann's thoughts at this time, to derive from his lifelong guiding leitmotiv, sacramental union with the loved one: as names, as dates, as initials; as numbers, as ciphers, as notes of music; in correspondence, in articles, in dedications, in programmes; in piano works, in songs, in duets; by him, by her, by both together; over and over again, year after year, in a bewildering profusion of alphabetical, numerical, cryptographical and finally and essentially - musical fantasy.
It is extremely fitting that the day which separated Clara from Eusebius - August 13 - should also have been, in 1837, the day of their reconciliation after estrangement. “On the day of Eusebius” wrote Schumann “we were betrothed.” But he still went on linking his names with hers in imagination long after she had changed her name to his in reality. Nor could the fictional saints rest from their labours even when the union was blessed in fact. All their eight children were little saints; virtual images of love and music, the goodness of life. Five of their names were Emil, Ferdinand, Elise, Felix and Eugenie.
 Erler, H. Robert Schumann’s Leben (1887) vol. i 233 et seq.
 Slater, E. & A. Meyer. Confinia psichiatrica (1959) vol 2 n. 2.
 ie the novelist J.P.F. Richter (1763-1825.
 Langer, S. Philosophy in a new way (1948) 19.
 for example “feierlichst erwählter… Clarus” or “Erregung und Felicitas".
 Chissell, J. Schumann (1956) 1.
 Dahms, w. Schumann (1916) 49.