40 Nigidius taught that the dii Penates were Neptune and Apollo, who once, on fixed terms, girt Ilium with walls. He himself again, in his sixteenth book, following Etruscan teaching, shows that there are four kinds of Penates; and that one of these pertains to Jupiter, another to Neptune, the third to the shades below, the fourth to mortal men, making some unintelligible assertion. Caesius himself, also, following this teaching, thinks that they are Fortune, and Ceres, the genius Jovialis, and Pales, but not the female deity commonly received, but some male attendant and steward of Jupiter.
Varro thinks that they are the gods of whom we speak who are within, and in the inmost recesses of heaven, and that neither their number nor names are known. The Etruscans say that these are the Consentes and Complices, and name them because they rise and fall together, six of them being male, and as many female, with unknown names and pitiless dispositions, but they are considered the counsellors and princes of Jove supreme. There were some, too, who said that Jupiter, Juno, and Minerva were the dii Penates, without whom we cannot live and be wise, and by whom we are ruled within in reason, passion, and thought.
As you see, even here, too, nothing is said harmoniously, nothing is settled with the consent of all, nor is there anything reliable on which the mind can take its stand, drawing by conjecture very near to the truth. For their opinions are so doubtful, and one supposition so discredited by another, that there is either no truth in them all, or if it is uttered by any, it is not recognised amid so many different statements.
Source: Against the Heathen (New Advent)