Image 1 Caption: Bertrand Russell (1872--1970), mathematician and philosopher, when relatively young.

See also St. Andrews History of Mathematic Archive: Bertrand Russell.

One of Bertrand Russell's points, when he was waxing philosophical---that impressed yours truly as a young person on a long car trip---was that super-pedantically, you could say there are almost NO real direct observations---nothing is directly knowable (or "observed"), except that "there is thinking"---which was probably noted by others too.

Image 2 Caption: Bertrand Russell (1872--1970) when relatively middle-aged.

Rene Descartes (1596--1650) assumed too much with cogito ergo sum---how does one know there is an "I" to think?

Howsoever, St. Augustine (354--430 CE):

You, who wish to know, do you know you are? I know it. Whence are you? I know NOT. Do you feel yourself single or multiple? I know NOT. Do you feel yourself moved? I know NOT. Do you know that you think? I do.
---St. Augustine (354--430 CE) Quotation from A History of Western Philosophy (1945), p. 355 by Bertrand Russell (1872--1970).

Images:

1. Credit/Permission: Anonymous photographer printed in Bertrand Russell, Justice in War-Time, 1916 (uploaded to Wikipedia by User:Spellcast, 2010) / Public domain.
   Image link: Wikipedia: File:Honourable Bertrand Russell.jpg.
   
2. Credit/Permission: Anonymous photographer, 1924 / (uploaded to Wikimedia Commons by User:DanielTom, 2013) / Public domain.
   Image link: Wikimedia Commons: File:Bertrand Russell in 1924.jpg.
   
3. Credit/Permission: Anonymous photographer, circa 1960 / Unknown, but it's just hotlinked and it's commonly found online.
   Image link: Your thought for today: Alas, now a dead link.