Might be in the wrong forum, but why is the first person singular used instead of the infinitive for Latin verbs in dictionaries?
Well, they’re both used, and because it defines the principle parts. Alone, you wouldn’t be able to tell that facere was a 3rd-IO verb, but seeing facio, facere let’s you know that it is. Similarily, unless a verb follows the usual pattern (like amo, amare, amavi, amatus, or audio, audire, audivi, auditus etc…) all four principle parts should be listed.
Yes, I know, but why not do is as in other languages where the first principle part is the infinitive and use that as the dictionary entry?
Yes, I’m interested in the lexicographical history of this convention. Let’s hope someone can elaborate on this.
I already explained thus, to differentiate between 3rd and 3rd-IO.
dicere, dixi, dictus
facere, feci, factus
How, besides memorization of every one, would you know that facere is 3rd-IO?
If I understand the question, it is not why do you need the first person singular, but why is that the one that is listed first. e.g., why not amare, amo, amavi, amatus?
Obviously it’s by convention, but why is the convention this way? I don’t have the slightest idea, by the way, but it is an interesting question.
Nah, the question was clearly stated to mean first person singular instead of infinitive. The order? Because the first is the simplest form out of the four.
Well, let’s restate the question: Why is the first peson singular the first of the entries? Well, it seems you answered it, cweb.
I suppose they are in the order that one would learn them. You also need the first principal part to distinguish between the 2nd and 3rd conjugations if macrons are absent, but otherwise you really only need it for the -io verbs (as cweb noted).
The infinitive is (to the Roman mind anyway e.g. historic infinitives) the basic form of the verb and for that reason should perhaps be listed first.
I agree. Explaining anything I use the infinitive as it seems right. Maybe that’s because I am a resurrected Roman boy with a lacking vocabulary I do not know 
There may not be any reason for it. Just a custom.
For myself, I would not like using a dictionary that did not list words this way. I’m far too accustomed to thinking of the first-person-singular as the “dictionary form” of a verb.
Does anyone know how far back this custom goes? It may have deep roots. The same thing is done in Greek dictionaries, by the way. Perhaps this custom began with the Greeks? Maybe the ancients thought of the first-person-singular as the “dictionary form.” I’m just guessing, I have no idea.
I have a vague recollection that in German books the infinitive is given instead of the first person singular.
Thucydides - did you mean infinitive first or infinitive only? The latter is absurd. But the reason, I suspect, that first person first is based on three assumptions. 1) first person singular present indicative has the word first in it, 2) It’s the simplest form, 3) you take am-o and then depending on its conjugation add the infinitive ending -are. Who knows, in particular, who cares?
I’ll agree it’s not the most important issue of our time, but I do think it is a curiosity. Fundamentally only one word is the “dictionary entry” while the others are listed along with it. For amo amare … it may not matter which you choose, but how did they decide that if I want to look up sum esse … that I need to turn to the ‘s’ section instead of the ‘e’ section? I agree that it seems more natural for a reference source to use the infinitive, but that is just a personal opinion.
but how did they decide that if I want to look up sum esse … that I need to turn to the ‘s’ section instead of the ‘e’ section?
Well, how did they decide that if I want to look up be in an English dictionary, I need to turn to the ‘b’ section instead of ‘a’ as in ‘am’?
The edition of Lysias’ Murder of Eratosthenes I used listed verbs in the infinitive form. I did find it curiously unsettling.