Pea? bean? pea?

Advised background music. Sonny Terry & Brownie McGee [I don't want no] Cornbread, peas, black molasses.

Legumes, anyone? One of our friends has gorn all vegan on us and invited us to dinner last week. Because the cook is English, one of the dishes involved carlins. carlings or carlin peas beans PEAS. As I hint, we had an argument robust discussion about the definition of, and difference between, peas and beans.

Well the quick answer is: whatever you're having yerself. Or like Humpty-Dumpty "When I use a word," Humpty Dumpty said, in rather a scornful tone, "it means just what I choose it to mean—neither more nor less." You say potahto, I say potayto. Garbanzo beans and chick peas, are after all, exactly the same thing.

For me, a helpful sort of scientific answer is to declare that peas - marrowfat; yellow split; snow; sugar-snap; mange-tout; and Birdseye frozen - are all members of the species Pisum sativum.

Another, more helpful definition is that peas is round, beans is longer. In microbiology, a similar gross morphology definition is between coccus (round) and bacillus (rod-shaped). Inevitably, there is a grey area in both classifications: ovoid-like. What you call a thing is contra Humpty-Dumpty important if you plan on communicating with other people about it. This is why science is quite particular about Latin binomers: bat or bear or bean are extremely imprecise lumpings of quite different things.

My contribution to the pea/bean dinner debate (apart from requiring a better definition of the terms before a useful answer was possible) was to say that afaik peas were all the same species but beans embraced a much wider genetic landscape. Some post-prandial research showed that this is true. All the commonloy edible pea/beans are in the family Fabaceae. Current phylogenetic consensus reports 6 tribes, 10 genera and 16 separate species within the family; thus:
Family Fabaceae

• Dalbergieae
• Arachis 
•  A. hypogaea peanut
• Cicereae 
• Cicer 
• C. arietinum chickpea 
• Fabeae
• Pisum
• P. sativum pea
• Millettieae
• Millettia
• M. pinnata Indian beechnut
• Phaseoleae
• Cajanus
• C. Cajan pigeon pea
• C. cajanwas carlins
• Glycine 
• G. max soybean
• Phaseolus
• P. lunatus lima bean
• P. vulgaris common bean incl kidney beans, haricot
• Vigna
• V. angularis adzuki bean
• V. mungo black gram
• V. radiata mung bean
• V. subterranea Bambara groundnut
• V. unguiculata cowpea, incl. black-eyed peas
• Vicieae
• Lens
• L. culinaris lentil
• Vicia
• V. faba broad bean

I hope that's helpful. Now I know.