But the reality is that 99% of the original source material is not relevant, will not be particularly rewarding, and may require a grad-level background to even understand many of the sentences. You want to read Hobbes Leviathan? Guess what, the only parts that matter today are a handful of pages. Same with Kant's Groundwork, Aristotle's Politics, etc.

The famous philosophers are (mostly) famous for their essential ideas, not the full lengths of their books.

For example: the majority of Hobbes' Leviathan is coming up with an entire theory of natural law, dealing with religion, etc. But none of that stuff was particularly noteworthy. The part that's relevant today was his original thoughts on the social contract, which is a very short part.

It's far more important to read textbooks, surveys, etc. to understand the ideas and be able to put each philosopher in context of who they were responding to and who responded to them.

Then if you're an academic or going to write a paper or something, sure go to the actual sources.

I say this having read tons of Plato, Aristotle, Seneca, Kant, Locke, Rawls, etc. I've needed to for my work, but I would never recommend reading them to someone just casually interested in philosophy. That's like telling someone who wants to learn how to program that they should be reading the C compiler source code.

Any summary material can only speak to the results of philosophy, while the primary sources when closely analyzed (like source code!) yield the process. Which will be more helpful if one wants to develop a "personal philosophy"? Working out the chains of reasoning or picking-and-choosing from a menu?

The primary works often just blatantly "state" things as if they were opinions. Then it's up to generates of scholars afterwards to argue about what the actual process of logical argument was.

Historical philosophy books aren't mathematical proofs. They don't necessary demonstrate process at all, it's not philosophy being "done" but just stated. Very often, it's precisely the summary works that situate the thinkers in a context that illuminates the process that led to their conclusions.

Also I don't know what you mean about "developing a personal philosophy"... I'm talking about academic philosophy here.

I actually agree that philosophical works are not mathematical proofs. But thought processes do not close over logic, they close over natural language. The point of reading primary texts is to interpret the text, to reverse out the thought process of what is being said through close reading and exposing hidden assumptions (i.e. hermeneutics). This isn't a strictly logical process, but it's common to all natural-language argumentation, including philosophy and law.

With that in mind, if Kant, Aristotle, Seneca and Rawls count as primary works – then what you're saying is false. They didn't state things in a vacuum; they all developed their views in a context, some of it from some base observations, much of it responding to interpretations of other points of view.

You have a point in that primary sources are not sufficient on their own, and scholars use their experience and erudition to understand these observations and fill in the context, particularly when the writing is otherwise more sparse. But neither would introductory texts be sufficient since they hide a lot of the tacit knowledge that could allow one to reason the way the author of a text reasoned when they produced their philosophical viewpoint.

That said if the person asking this question has no interest in being an academic, your advice is horrible. Imagine reading about Plato but never reading a dialogue, never getting a sense of being in a room with Socrates. Imagine reading about the death of God but not a single sentence of Nietzsche’s maddeningly beautiful prose. You need to engage with real philosophy.

At the very least, if you're going to go for primary sources, find a guide or lecture to lead you through it. Of course primary sources are important to read in philosophy, but in my personal opinion, I would recommend going for an introductory text first and picking primary sources to read from there.

Telling someone "read the Nicomachean Ethics by Aristotle" will lead someone to just pick up any copy of the Nicomachean Ethics. There are a lot of translations of the Nicomachean Ethics and they are not all equal. They range from very good translations, to idiosyncratic readings of the text, to flat out bad translations.

Beyond poor translations, the ancients that you've recommended are good to start with and the recommendation to read the primary sources of them is just fine. Those texts are easy to digest without having a formal background in philosophy.

But for other philosophers (e.g. Nietzsche, Kierkegaard, Hegel), telling someone to just "pick up one of their primary texts" is a disaster. It will either (a.) be complete gibberish to the reader without any context and they may just give up, or worse, (b.) they think they'll understand something without the proper context and spew nonsense in regards to that philosopher (this is why there are so many bad readings of Nietzsche).

So depending on what you're recommending, primary sources can be good, but in my experience, primary sources aren't good most of the time. Moreover, if someone is interested in a specific field like the OP is, then having a good secondary source can be extremely helpful to give someone an overview and proper understanding of the topic.

This is why you'll want to read those primary sources as part of a class, instead of just trying to go it alone.

But even that won't save you from "bad readings" of Nietzsche or any other philosopher, as plenty of experts disagree on what he meant. He, like some other philosophers, just wrote in a way that doesn't have one obvious meaning that everyone can agree on. With experience and study, you can make up your own mind, which will be better than swallowing some other person's pre-digested interpretation of him.

You might have an easier time understanding a secondary source's interpretation of Nietzsche, but that doesn't mean that you understand Nietzsche.

And of course there are many interpretations of Nietzsche and there's reasonable disagreement on what he said. You're right that a secondary source or taking a class doesn't "save you from bad readings" of him, but it's still better than trying to go it alone.

There are many flat out wrong interpretations of him, and someone like a professor or a secondary source can definitely help avoid common misunderstandings and pitfalls when trying to read him.

That really depends on who you read. If you read only a secondary source instead of the primary source, and that secondary source happens to have misinterpreted the primary, you're going to be misled.

If you read the primary source you're at least going to have the chance to make up your own mind, even if it's difficult to do so... and even if you can't, you might at least see that what the primary source actually says might not be as straightforward and obvious as the secondary source maintains.

But please don't think I'm against secondary sources altogether. They can be a useful adjunct to reading the primary sources. Ideally, though, you'd have multiple secondary sources (ones that disagree with one another), so you don't fall in to some one person's reality tunnel.

This is especially important in philosophy. I can't count the number of times I've read secondary sources which I consider to have completely misunderstood the primary sources they were commenting on, and how frequently secondary sources disagree with one another (especially on the more "difficult" philosophers).