i 
[Communicated for the Spy ] 
The New Theory. 
Letter from dr. mitchill to cap¬ 
tain SYMMES. 
New York, 19 ih Sept. 1819. 
John Cleves Symmes, esqr. 
* SIR, 
I owe you an acknowledgment for your 
several late communications on the hol¬ 
lowness of our earth, and its openings at 
"the poles. I give you great credit for the 
ingenuity and originality displayed in sup¬ 
port of your hypothesis. 
You must not be alarmed because I em¬ 
ploy the word hypothesis. By it is under¬ 
stood, any position or point a logician 
states and proclaims his intention to main¬ 
tain. When this is a necessary and unde* 
triable thing, as in the elements of geome» 
try, it is called a postulate. When it is not 
so evident, but, on the contrary, may be 
fairly argued pro and con, as in academic 
exercises, they name it a thesis; and when 
assumed as a philosophical dogma, upon 
which men exercise their reasoning pow¬ 
ers, it is rightly denominated an hypothesis. 
Hypotheses are of two kinds, correct 
and erroneous. They are in the nature of 
conjectures, which are not necessarily 
ci'her true or false; but may be one or 
the other, according to their character 
and value. 
Theories are of different denominations, { 
inasmuch as they are the deductions of a 
rational nature from acknowledged and 
established premises. The employment of 
the mind in theorizing is one of its noblest 
exercises. The best theorist affords the 
fairest display of intellectual power in 
man. In politics, in physics, and in the 
other departments of science, just theory, 
implying a perfect acquaintance with prac¬ 
tice, give most exalted views of the human 
understanding. 
