Brothers and sisters have I none, but that man's father is my father's son.

logic/kinship Added material

Problem

Brothers and sisters have I none, but that man's father is my father's son.
Who is that man?

That man's mother is my mother's mother-in-law.  Who is that man?

My son's father is your father's only child.  Who am I?

What is the simplest way in which two people can be an uncle to each other?

Show solution

Your son.

Your father. [Henry E. Dudeney, A Puzzle-Mine, Nelson, London, c1918, #317]

Your wife. [Peter Storme (pseudonym for James Neuman) and
  Paul Stryfe (pseudonym for Philip van Doren Stern), How to Torture
  Your Friends, Simon and Schuster, New York, 1941, #89]

Sons of two men who married each other's mothers. [Henry E. Dudeney,
  A Puzzle-Mine, Nelson, London, c1918, #305]

Additional material

[ Editor: on the first question, posed as a man holding a photo who
says "Brothers and sisters have I none, this man's father is my
father's son": ]

There are several solutions to this problem:
1. the man's son who stated the riddle
2. if a sibling has died it may be a nephew
3. it could be an adopted child
4. when he said son it could have meant 'god-son'
5. if the father had other children or remarried to another woman it
could be one of their children
    [my father remarried, but i don't consider their children my
    brothers or sisters]

-- David Mitchell <mitchell@jersey.net>

Another kinship puzzle, posted in 1998:

    Clothaire, Chris, and Timmy are related to one another, but not
    incestuously.

    1.  Among the three are Clothaire's father, Chris' only daughter,
        and Timmy's sibling.

    2.  Timmy's sibling is neither Clothaire's father nor Chris'
        daughter.

    Which one is a different sex from the other two?

    -- Edward Cooper <crystale@erols.com>

Timmy is the one who is a different sex from the other two.

-- Kevin Green <kdg977@dial.net>

There is only one way to meet all the criteria; Clothaire's father is
Chris, Chris' only daughter is Timmy which makes Clothaire Timmy's
sibling.  Chris is father to both Clothaire and Timmy but Chris only has
one daughter which means that Clothaire must be Chris' son.  We have two
males and one female.  The one of a different gender is Timmy.

-- Glenn C. Rhoads <rhoads@paul.rutgers.edu>

[ Editor: on the first question, asked in 1996 as a man who stops at a
newspaper stand, stares at a photo and says "Brothers and sisters I
have none, but that man's father was my father's son", after a
disagreement about the answer on a BBS: ]

Well, my reasoning is as follows.  If I have no sisters or brothers,
then my father's son must be me.  So the man says, in effect, "that
man's father is me."  So, the photo must be of his son.

Since this seems easy, and you state there was a big disagreement on
the BBS, I'll try to see where a tricky person would dispute this
argument:

1.  The man was not talking about anyone in the photo when he made this
statement.  Maybe he was talking about the newspaper vendor or the
person who took the photo (since photographer credits usually are given
alongside newspaper photos).  But then there is no unique way to answer
the riddle.

2.  If we argue that half-sisters and half-brothers are not sisters and
brothers, then "my father's son" could refer to my half-brother, and
then the man in the photo could be my nephew.  But that seems like a
stretch, at least to me.

3.  When the man says "that man's father was my father's son" notice
that the past tense is used.  Thus this should translate to "that man's
father was me."  Now, fatherhood does not lapse with time, so the use
of the past tense is curious.  Perhaps the man in the photo is dead.
Failing that, perhaps the word "father" is used in an unusual sense.
One such sense is that of a priest.  Perhaps the claim is "that man's
priest was me."  Perhaps the speaker is a defrocked member of the
clergy, and the man is the photo used to be in his church.  But then
the phrase "my father's son" equally could refer to a religious
relationship, and again the riddle has many possible answers.

That's all I can come up with off the top of my head.  I hope this
helps.  I'd be interested in any other disputable items in this riddle.

-- Chris Cole <chris@questrel.com>