Socrates asks Cephalus "whether your fortune was for the most part inherited or acquired by you?"
In talking about "fortune" the immediate conversation is about physical wealth, money, geld, filthy lucre. But the metaphorical fortune to which Socrates is alluding is actually the spiritual bounty which one reaps through justice.
Cephalus replies that he has acquired, or worked for his fortune (meaning physical money) and that he is "midway between my father and grandfather: for my grandfather, whose name I bear, doubled and trebled the value of his patrimony, that which he inherited being much what I possess now; but my father Lysanias reduced the property below what it is at present: and I shall be satisfied if I leave to these my sons not less but a little more than I received."
Socrates has, in the back of his mind, the fortune of spiritual insight, not that of physical wealth. Consequently Cephalus' answer is to be read as "My grandfather worked for justice, my father squandered that good inheritance, I have had to regain a great deal of spiritual wealth through trying to be just." Essentially, Cephalus has experienced justice without guidance from his parents - like a child of the 60s cut off from his (spiritual) inheritance.
Socrates then suggests that people who inherit wealth (spiritual, or cultural wealth) are indifferent about it; they treat it as though it has always been this way. Those who work for virtue undstd the importance of it and value it more - but also talk incessantly about their own goodness, about God, about spiritual things (Jesus this, and Jesus that).
"And hence they are very bad company, for they can talk about nothing but the praises of wealth."
When Socrates asks what the greatest blessing of wealth is he is referring metaphorically to the greatest blessing of spiritual wealth; the blessing of justice. Cephalus' answer, although again about physical wealth, relates directly to spiritual wealth simultaneously. He says he "could not expect easily to convince others" but
"when a man thinks himself to be near death, fears and cares enter into his mind which he never had before; the tales of a world below and the punishment which is exacted there of deeds done here were once a laughing matter to him, but now he is tormented with the thought that they may be true: either from the weakness of age, or because he is now drawing nearer to that other place, he has a clearer view of these things; suspicions and alarms crowd thickly upon him, and he begins to reflect and consider what wrongs he has done to others. And when he finds that the sum of his transgressions is great he will many a time like a child start up in his sleep for fear, and he is filled with dark forebodings. But to him who is conscious of no sin, sweet hope, as Pindar charmingly says, is the kind nurse of his age."
One of the greatest spiritual benefits of justice is peace of mind that one is not headed to hell (or Tartarus in this case) but is destined for that happy Elysium which the poets and priests foretell to us.
Cephalus then suggests that having physical wealth greatly enhances this ablty to be just, and thus peace of mind (here he conflates virtue with physical wealth, as does Adam Smith later on).
The great benefit, then, of justice, the "trade of spirituality" seems to be peace of mind - happiness. But Socrates wants to know what the trade of spirituality is; how can he, or any man, achieve this attractive happiness? what is the method, or technique?
To this Cephalus replies that justice is simply speaking the truth and repaying your debts: he gives a very formulaic opinion of justice, shallow, a series of actions rather than internal disposition. Socrates uses the parallel of a trade - namely arms dealing or banking in weapons - to show that justice can't be as shallow or simple as Cephalus seems to think:
"but as concerning justice, what is it? --to speak the truth and to pay your debts --no more than this? And even to this are there not exceptions? Suppose that a friend when in his right mind has deposited arms with me and he asks for them when he is not in his right mind, ought I to give them back to him? No one would say that I ought or that I should be right in doing so, any more than they would say that I ought always to speak the truth to one who is in his condition."
If justice is no more than a trade, a techne, then anyone could & would do it, the gods would be easily duped, and more people would be justice than are actually just in the world.
Consequently, Justice has to be more than a simply techne; "speaking the truth and paying your debts is not a correct definition of justice," he says - at which point the head (Cephalus) leaves the discussion and his son, Polemarchus, the war leader, takes over.