It may be worse for others; but for him and you there is no dread. He is a noble fellow; and let me tell you from experience of men, that one who would do as he did in going down that wall and to that room—ay, and going a second time—is not one to be injured in permanence by a shock. His brain and his heart are all right; this I swear, before I have even seen him; so be at rest.
This is another one of those weird fucking instances where Bram Stoker got to the "right" answer in the wrongest conceivable way.
The argument Van Helsing is making here is that Jonathan is too strong of character to be permanently traumatized.
Obviously that is not how PTSD works,
I'm not going to pretend otherwise, so be at ease, my friend.
Generally speaking, when PTSD alters your brain structure, those changes are permanent, or long enough lasting that they may as well be. However, as anyone who has undergone half-decent post-traumatic care can attest, the debilitating symptoms of PTSD can ease over time as you learn how better to avoid, control, and recover from triggers, and develop better coping skills.
One common (though far from universal) predictor of how severely an event will traumatize a person is related to autonomy. The freedom and ability to make your own choices. The less autonomy a person is able to exercise during and after an traumatic event (or, the more frequently their autonomy is overridden by the situation), the worse the trauma symptoms tend to be.
In contrast, a lot of early therapeutic steps in treating PTSD involve reclaiming autonomy. This looks different for different people, because it obviously has to be individualized. But, common examples of exercising autonomy after trauma include re-framing the trauma through art (writing, reading, painting, whatever) so that the victim can, in a sense, control the "story" of the traumatic event even though they could not control the event itself.
By sheer coincidence, Jonathan Harker has lucked into probably the best case scenario.
His autonomy during his imprisonment was constantly degraded in tiny and massive ways, from controlled sleep schedule changes to forced denial of grooming habits straight up through undressing and implied penetration without consent.
However, he persisted in making decisions and carrying them out, even in spite of these controls. And eventually one of those decisions saved his life. This can easily be turned into a coping skill. He seems not to have lost the ability to make decisions for himself, thus "that step" (as it were) can be "skipped." And since the "steps" had not been invented yet, that definitely puts Jonathan in a better position for recovery.
But let's loop back around to therapies for trauma. Jonathan also happens to have taken a critical step in enforcing his autonomy post-event, too. By entrusting Mina with his journal, he made the conscious decision to let her be his guide. That too is a type of reclamation of autonomy over the story of his trauma. Yes, it means he isn't "making the decisions" himself, but that is a choice he made and is continuing to make each day, safe in the knowledge that if he changes his mind, Mina will still trust him.
That right there combines both autonomy and stability, which also enable one to learn PTSD skills more quickly.
Combine that with the fact that his wife is probably the most competent caretaker short of Mary Poppins and you have a basically ideal candidate for recovery.
Not in any way because of the weird shit V.H. was saying. Just as a coincidence.
And I think to some degree, Stoker likely recognized that pattern, because it plays out pretty regularly in real life. Just, he blamed it on "inherent moral fortitude" rather than "the external support offered to middle-to-upper-class men is so robust and the freedom of choice offered to them so complete that a man in Jonathan's position is simply much more likely to recover than any working class or otherwise marginalized person in this situation."
(Surprise! It was a post about classism all along!)