How old are people when they first start being able to reason?
I vaguely recall reading somewhere in the psychological literature on the development of reasoning skills that kids are able to reason as early as 4. However even that may underestimate how early these skills emerge.
Our daughter is 3 years and one month old. Here’s a conversation we had this evening.
Lillian (singing): “I’m the king of the castle, and you the dirty bascal.”
Us: “Its rascal, not bascal.”
“No, its BASCAL.”
“OK, well, grown-ups say rascal. Little babies say bascal.”
Lillian ponders for a moment, then smiles. “I’m a baby!”
She had performed the following inference:
Babies say bascal.
I say bascal.
Therefore, I’m a baby.
This of course is an invalid inference; she might be something other than a baby which also says bascal.
However logical perfection is a lot to ask of a three year old. In context, despite not being deductively valid, it wasn’t a bad inferential move to make.
She’s been making these sorts of inferences for a while – indeed, since well before she was three. And explicit or verbalised inference was preceded by patterns of behavior which seemed to indicate an implicit understanding of certain inferential patterns.
So we can say the following with certainty: Reasoning abilities of a rudimentary kind are, at least sometimes, exhibited by humans before they turn 3.
Tim van Gelder 
Tim,
She must be that good from her mother’s side. Like the good looks… ;-)
Pierre
Tim,
Having four kids of my own, I can’t help but wonder if yours was looking for an excuse to say ‘bascal’ and found one. Perhaps more of a manipulation than a logical insight… Kids are so good at it. A logical possibility.
Peter
It’s possible that your daughter’s inference was indeed valid since it’s plausible that when you said “Little babies say bascal” you intended that *only* little babies say bascal. Thus, she would have correctly inferred that because shes says bascal and only little babies say bascal, she is a baby. (I know the post is probably intended to be a whimsical, but I thought you would appreciate the suggestion.)
Another possibility, along the lines of Peter’s suggestion, is that Lillian seized on an opportunity to say she was a baby. My 3-1/2 year old son enjoys such opportunities.
Yet another possibility is that she’s fumbling towards “dirty bastard” which after all is what grown-ups REALLY say. :-)