have never been on the moon than that I have never been in Bulgaria? Why am I so sure? Well, I know that I have never been anywhere in the neighbourhood - for example I have never been in the Balkans.
 "I have compelling grounds for my certitude." These grounds make the certitude objective.
 What is a telling ground for something is not anything I decide.
 I know = I am familiar with it as a certainty.
 But when does one say of something that it is certain?
For there can be dispute whether something is certain; I mean, when something is objectively certain.
There are countless general empirical propositions that count as certain for us.
 One such is that if someone's arm is cut off it will not grow again. Another, if someone's head is cut off he is dead and will never live again.
Experience can be said to teach us these propositions. However, it does not teach us them in isolation: rather, it teaches us a host of interdependent propositions. If they were isolated I might perhaps doubt them, for I have no experience relating to them.
 If experience is the ground of our certainty, then naturally it is past experience.
And it isn't for example just my experience, but other's people's, that I get knowledge from.
Now one might say that it is experience again that leads us to give credence to others. But what experience makes me believe that the anatomy and physiology books don't contain what is false? Though it is true that this trust is backed up by my own experience.
 We believe, so to speak, that this great building exists, and then we see, now here, now there, one or another small corner of it.
 "I can't help believing..."
 "I am comfortable that that is how things are."
 It is quite sure that motor cars don't grow out of the earth. We feel that if someone could believe the contrary he could believe everything that we say is untrue, and could question everything that we hold to be sure.
But how does this one belief hang together with all the rest? We should like to say that someone who could believe that does not accept our whole system of verification.
This system is something that a human being acquires by means of observation and instruction. I intentionally do not say "learns".
 After he has seen this and this and heard that and that, he is not in a position to doubt whether...
 I, L