Reportedly, Alan Kay is the inventor of the term "object oriented". And he is often quoted as having said that what we call OO today is not what he meant.
For example, I just found this on Google:

I made up the term 'object-oriented', and I can tell you I didn't have C++ in mind
-- Alan Kay, OOPSLA '97

I vaguely remember hearing something pretty insightful about what he did mean. Something along the lines of "message passing".
Do you know what he meant? Can you fill in more details of what he meant and how it differs from today's common OO? Please share some references if you have any.

TL;DR - Alan Kay wrote in 2003 that:

OOP to me means only messaging, local retention and protection and hiding of state-process, and extreme late-binding of all things.
It can be done in Smalltalk and in LISP. There are possibly other systems in which this is possible, but I'm not aware of them.

http://www.purl.org/stefan_ram/pub/doc_kay_oop_en
------- Full source, for context:

Date: Wed, 23 Jul 2003 09:33:31 -0800   To: Stefan Ram [removed for
privacy]   From: Alan Kay [removed for privacy]   Subject: Re:
Clarification of "object-oriented"
Hi Stefan --
Sorry for the delay but I was on vacation.
At 6:27 PM +0200 7/17/03, Stefan Ram wrote:

Dear Dr. Kay,
I would like to have some authoritative word on the term
"object-oriented programming" for my tutorial page on the
subject. The only two sources I consider to be "authoritative"
are the International Standards Organization, which defines
"object-oriented" in "ISO/IEC 2382-15", and you, because,
as they say, you have coined that term.

I'm pretty sure I did.

Unfortunately, it is difficult to find a web page or source
with your definition or description of that term. There are
several reports about what you might have said in this regard
(like "inheritance, polymorphism and encapsulation"), but
these are not first-hand sources. I am also aware that later
you put more emphasis on "messaging" - but I still would like
to know about "object oriented".


For the records, my tutorial page, and further distribution
and publication could you please explain:
When and where was the term "object-oriented" used first?

At Utah sometime after Nov 66 when, influenced by Sketchpad, Simula,
the design for the ARPAnet, the Burroughs B5000, and my background in
Biology and Mathematics, I thought of an architecture for programming.
It was probably in 1967 when someone asked me what I was doing, and I
said: "It's object-oriented programming".
The original conception of it had the following parts.

I thought of objects being like biological cells and/or individual computers on a network, only able to communicate with messages (so
messaging came at the very beginning -- it took a while to see how to
do messaging in a programming language efficiently enough to be
useful).

I wanted to get rid of data. The B5000 almost did this via its almost unbelievable HW architecture. I realized that the
cell/whole-computer metaphor would get rid of data, and that "<-"
would be just another message token (it took me quite a while to think
this out because I really thought of all these symbols as names for
functions and procedures.

My math background made me realize that each object could have several algebras associated with it, and there could be families of
these, and that these would be very very useful. The term
"polymorphism" was imposed much later (I think by Peter Wegner) and it
isn't quite valid, since it really comes from the nomenclature of
functions, and I wanted quite a bit more than functions. I made up a
term "genericity" for dealing with generic behaviors in a
quasi-algebraic form.

I didn't like the way Simula I or Simula 67 did inheritance (though I thought Nygaard and Dahl were just tremendous thinkers and
designers). So I decided to leave out inheritance as a built-in
feature until I understood it better.


My original experiments with this architecture were done using a model
I adapted from van Wijngaarten's and Wirth's "Generalization of Algol"
and Wirth's Euler. Both of these were rather LISP-like but with a more
conventional readable syntax. I didn't understand the monster LISP
idea of tangible metalanguage then, but got kind of close with ideas
about extensible languages draw from various sources, including Irons'
IMP.
The second phase of this was to finally understand LISP and then using
this understanding to make much nicer and smaller and more powerful
and more late bound understructures. Dave Fisher's thesis was done in
"McCarthy" style and his ideas about extensible control structures
were very helpful. Another big influence at this time was Carl
Hewitt's PLANNER (which has never gotten the recognition it deserves,
given how well and how earlier it was able to anticipate Prolog).
The original Smalltalk at Xerox PARC came out of the above. The
subsequent Smalltalk's are complained about in the end of the History
chapter: they backslid towards Simula and did not replace the
extension mechanisms with safer ones that were anywhere near as
useful.

What does "object-oriented [programming]" mean to you?
(No tutorial-like introduction is needed, just a short
explanation [like "programming with inheritance,
polymorphism and encapsulation"] in terms of other concepts
for a reader familiar with them, if possible. Also, it is
not neccessary to explain "object", because I already have
sources with your explanation of "object" from
"Early History of Smalltalk".)

(I'm not against types, but I don't know of any type systems that
aren't a complete pain, so I still like dynamic typing.)
OOP to me means only messaging, local retention and protection and hiding of state-process, and extreme late-binding of all things.
It can be done in Smalltalk and in LISP. There are possibly other systems in which this is possible, but I'm not aware of them.
[Also,] One of the things I should have mentioned is that there were
two main paths that were catalysed by Simula. The early one (just by
accident) was the bio/net non-data-procedure route that I took. The
other one, which came a little later as an object of study was
abstract data types, and this got much more play.
If we look at the whole history, we see that the proto-OOP stuff
started with ADT, had a little fork towards what I called "objects"
-- that led to Smalltalk, etc.,-- but after the little fork, the CS establishment pretty much did ADT and wanted to stick with the
data-procedure paradigm. Historically, it's worth looking at the USAF
Burroughs 220 file system (that I described in the Smalltalk history),
the early work of Doug Ross at MIT (AED and earlier) in which he
advocated embedding procedure pointers in data structures, Sketchpad
(which had full polymorphism -- where e.g. the same offset in its data
structure meant "display" and there would be a pointer to the
appropriate routine for the type of object that structure represented,
etc., and the Burroughs B5000, whose program reference tables were
true "big objects" and contained pointers to both "data" and
"procedures" but could often do the right thing if it was trying to go
after data and found a procedure pointer. And the very first problems
I solved with my early Utah stuff was the "disappearing of data" using
only methods and objects. At the end of the 60s (I think) Bob Balzer
wrote a pretty nifty paper called "Dataless Programming", and shortly
thereafter John Reynolds wrote an equally nifty paper "Gedanken" (in
1970 I think) in which he showed that using the lamda expressions the
right way would allow data to be abstracted by procedures.
The people who liked objects as non-data were smaller in number, and
included myself, Carl Hewitt, Dave Reed and a few others -- pretty
much all of this group were from the ARPA community and were involved
in one way or another with the design of ARPAnet → Internet in
which the basic unit of computation was a whole computer. But just to
show how stubbornly an idea can hang on, all through the seventies and
eighties, there were many people who tried to get by with "Remote
Procedure Call" instead of thinking about objects and messages. Sic
transit gloria mundi.
Cheers,
Alan Kay

