Those who cannot remember the past are condemned to repeat it. Who said that? Let me scroll back in Messages to find the exact person. (Ah, yes, George Santayana.)
One of iChats nicest features was the way it allowed you to sift through the transcripts of past chats; you always had a record of the promises you made for work (or in the rest of life). Although the new Messages app in Mountain Lion doesnt do away with transcripts, it repackages the feature in a way that many iChat veterans find obscure.
The key difference is embedded in the Messages interface: Chats are no longer things that live in their own tabbed pop-up windows. Instead, the app organizes conversations in the main Messages window: In that window (the one labeled Messages), chats appear in a conversation sidebar on the left. Entries in this list represent both active chats with a contact (across all the services they may use) and the historical record of those chats, if youve chosen to store it.
In iChat, you could set an option to save a transcript for every chat; that option is preserved but relocated in Messages. Under Messages > Preferences in the General view, check Save history when conversations are closed. If this option remains uncheckedits default settingno transcripts are saved. Check it, and the app writes all conversations to disk. (If the option is unchecked, quitting the program doesnt delete all record of a current conversation. But that record will be deleted when you close a conversation using the X button that appears when you hover over it with your pointer, or when a conversation is selected and you choose File > Close Conversation.)
If you have set up Messages with Apples iMessage system so that it uses the same Apple ID youre using on other devices and computers, the transcripts of conversations you had on those other devices will also be saved. For other chat services, such as AOL Instant Messenger, the text recordings are saved only on the machine from which you conducted the chat.
You can interact with conversation histories through the Messages sidebar in two ways.
Scroll back: For a given sessionor, with saved transcripts, all sessions everyou can select a contact in the conversation sidebar and then start scrolling backward (upward) to go back in time. The Messages app automatically loads previous days and conversation sections as necessary.
Search: Click the Search field and type in a term. Messages applies the term against any contact in the sidebar and all stored history for each contact, and hides contacts for which it finds no match. Use Command-G or Edit > Find > Find Next to cycle through matches in the transcripts. Command-Shift-G and Find Previous let you reverse direction.
Scrolling backward and searching both require that the contact with whom you had the chat youre looking for is in the conversation sidebar. If (for whatever reason) the contact isnt listed there, you must start a new conversation with him or her before you can see a transcript of your previous chats. Once you start that new conversation, the transcripts will light up in the chat section of the window.
(Tip: If a contact isnt in the conversation sidebar on the left, check the File > Recent Items menu; if the person is there, you can select that name to open a transcript without starting a new chat.)
You can find the files containing these transcripts with a bit of effort. In a Finder window, start typing chat in the field; the option Chat transcript will appear. Click that, and the Finder window will show all matching transcripts. You can then add a contacts name to your search terms to drill down more precisely. (The transcripts preview in Spotlight, but can be far too small to view.)
You can also examine individual files in the Finder by navigating to the hidden Library folder. In the Finder, choose Go > Go To Folder and enter ~/Library/Messages/Archive/. Messages sticks chat transcripts here, organizing them into folders by date; within those folders, transcripts are sorted into individual files by contact name. Any contact may have multiple files split up across a day by time, as well. Messages uses the same .ichat file format as its predecessor did. Double-click a file, and it will open in Messages as a view-only conversation. (The files have the contacts name, the date, and then the 24-hour-clock time stamp from that particular slice.)
Attachments, such as photos or any other files sent via Messages to another party, are also saved when you opt to keep transcripts. You can find them in ~/Library/Messages/Attachments/ sorted according to an internal numbering and naming system.
What you can do with conversations
Although you cant export an individual conversation or even part of one, you can copy and paste it in a relatively plain format. In a conversation or an opened transcript file, drag to select text across messages, or click a message and then extend via Shift-click (for contiguous selections) or Command-click (for noncontiguous).
Right-click any selected message, and select Copy or choose Edit > Copy, and then you can paste the results (with bold formatting for names, and messages indented below each chat participants name). Choosing Forward from that menu takes the formatted message or messages and creates a new empty conversation with that selection as the first message to send. You can also print the results (File > Print) and preserve its original look; I recommend printing to PDF, as conversations could consume a lot of paper.
Although you have these files neatly stowed away, you might decide to purge some of them at a later date. You can certainly go through the Archive folder and delete them, but you have two other options: surgical and nuclear.
In the surgical approach, while viewing a conversation in Messages, you can click a message balloon and then right-click it to choose Delete (or press the Delete key) to bring up a prompt to remove it. This action deletes entries from the transcript files and from the active conversation window.
If you prefer the nuclear alternative, select a conversation and choose Edit > Delete Transcript. The Messages app wont just delete what youre looking at on screen. Rather, it will go back through every stored chat file for that contact and delete all of them from diskwithout a prompt! (Thats what backups are for.)