We saw that Locke had several arguments against the notion of innate ideas and they went something like this:

If any idea were innate in all human minds then surely certain ideas would be universally accepted and agreed on. But they’re not.

Locke says that, 

to imprint anything on the mind without the mind’s perceiving it seems to me hardly intelligible. So if children and idiots have souls, minds, with those principles imprinted on them, they can’t help perceiving them and assenting to them. 

In other words it makes no sense to speak of an idea being in our minds without us being aware of it. How can we have ‘understandings’ that we don’t understand. Surely the whole point of an idea is that it is ‘had’. (As in, “I’ve just had an idea!” said Clive.)

Locke attacks saying this the argument that we only come to knowledge of these ‘innate’ ideas when we develop our ‘reason’, saying that it is non-sensical and seems circular’ because in order to know innate ideas we have to have reason, but the evidence that we have ‘reason’ is our knowledge of innate ideas. Doh!

This attack on innate ideas was revolutionary in Locke’s day and has profound implications: if a person is ‘blank slate’ then we can draw anything on it. This makes politics, education and the whole nature of a society fundamental to the kind of human beings it produces. This is the old nature/nurture debate which seems to rumble on forever without ever really getting anywhere, usually because the people debating it have political axes to grind and are not actually open to discussion.