Some scholars explain this 以 (이) by saying that it is used with such “special” adjectives as 足 (족) and 可 (가) when 足 and 可 are followed by intransitive verbs and by transitive verbs being used in the active voice. They further explain that 以 is omitted when 足 and 可 are followed by transitive verbs being used in the passive voice. Though that may be the result of using such adjectives, I suspect the ancient Chinese were not thinking about transitive and intransitive and passive and active when they wrote their sentences, so here is my simpler theory:
When 以 (이) follows adjectives like 足 (족) and 可 (가), it is being used with its verbal meaning of “to use.” Therefore, the clause “吾力足以擧三千斤 (오력족이거삼천근)” literally translates as follows:
“My (吾) strength (力) is enough (足) to be used (以) to lift (擧) 3,000 (三千) pounds (斤).”
孟子曰, 有人曰, 吾力足以舉三千斤, 而不足以舉一羽, 王信之乎, 可知一羽之不舉, 爲不用力也, 是其不爲, 而非其不能也.
Mencius (孟子) said (曰), “[Suppose] someone (有人) were to say (曰), ‘My (吾) strength (力) is enough (足以) to lift (舉) three thousand (三千) pounds (斤), but (而) is not (不) enough (足以) to lift (舉) one (一) feather (羽)’; [would] your Majesty (王) believe (信) him (之乎)? It would be obvious (可知) the one (一) feather’s (羽之) not (不) being lifted (舉) would be because (爲) [he] is not (不) using (用) [his] strength (力也). This (是) [is] his (其) not-doing (不爲), and (而) not (不) his (其) being incapable (不能).