The prevailing argument during Plato's time was that "true" knowledge, or ἐπιστήμη/epistēmē is the byproduct of metaphysical contemplation. See Aristotle's Metaphysics Book 12.1072b
Hence it is actuality rather than potentiality that is held to be the divine possession of rational thought, and its active contemplation is that which is most pleasant and best.
Also Timaeus 51d:
First then, in my judgment, we must make a distinction and ask, What is that which always is and has no becoming; and what is that which is always becoming and never is? That which is apprehended by intelligence and reason is always in the same state; but that which is conceived by opinion with the help of sensation and without reason, is always in a process of becoming and perishing and never really is. Now everything that becomes or is created must of necessity be created by some cause, for without a cause nothing can be created. The work of the creator, whenever he looks to the unchangeable and fashions the form and nature of his work after an unchangeable pattern, must necessarily be made fair and perfect; but when he looks to the created only, and uses a created pattern, it is not fair or perfect. […] As being is to becoming, so is truth to belief.
The distinction between belief and objective reality is important here. The Theory of Forms in the Republic are a great example of this distinction as well: only non-physical forms represent accurate reality. The sensible world (physical/perceived world) versus the intelligible world (contemplation and lover of beauty).
Loosely translated, ἐπιστήμη refers to knowledge, comprehension, or science.
This view was incompatible with following your "senses."
Plato defines those who follow their senses (people who depend on their sense-perception for knowledge) as lovers of sights and sounds
Republic: Book V: 471C-505B
The lovers of sounds and sights, I replied, are, as I conceive, fond
of fine tones and colors and forms and all the artificial products
that are made out of them, but their minds are incapable of seeing or
loving absolute beauty.
Notice the end of that Timaeus quote. This touches on Plato's second rejection of knowledge in the Theatetus: that knowledge is justified true belief. (I recommend reading Gettier's account for a better sense of this).
Specifically, your question is answered in Book I of the Theatetus, where Plato argues that knowledge (ἐπιστήμη) is incompatible with perception. For Plato, there are two criteria for knowledge: that it is infallible, that it is what is (οὐσία: ousia).
Now, it is important to know the philosophical context of why Plato makes this claim. Prior to Plato, there were two prevailing doctrines of knowledge. Both were argued that knowledge is perception.
These were: (1) Protagoras's claim that man is the measure of all things, and (2) Heraclitus's claim that everything is in flux. For Heraclitus, nothing stays the same, and thus neither does perception. What happens when the events of the physical world meets you (the arbiter of how the world appears) and then change? Both theses support that knowledge=perception, because perception seems to meet both criteria; it is what is (οὐσία), and assuming your perception/the appearance is correct, it is infallible.
Naturally, Plato disagrees with this:
Whether Heraclitus of Protagoras is right about how the world ultimately is does not change what knowledge is.
How does he achieve this? Through cognitive access. For example, to cognitively access a light being off is an immediate inference to "on-ness". The light being on means it's not off and vice versa. Thus, Plato argues, what is not requires some sort of recognition of what is.
In relation to knowledge and sense-perception, we need to be able to differentiate between things that are i.e. sameness, difference, beautiful, ugly; in order to possess knowledge. Perception is inconclusive and cannot be knowledge because for Plato, to have knowledge you require ta koina (that which is objective), versus ta idia (that which is subjective. This is Protagoras's claim).
In The Digression of Theatetus: 172c1-177b7, Plato distinguishes heavily between "the philosopher" and "the rhetorician" (This is Protagoras again) and demonstrates why it's beneficial to be the former.
The Digression is an allegory for what is ethically at sake in abstract conversations and dilemmas where perception cannot help us. You cannot deduce through senses alone whether or not it's right to be virtuous or wicked. Thus we cannot make a decision about what account of knowledge we must accept without accepting all these others claims in morality. In 176c for example, we find that the difference between justice and injustice is the difference between knowledge and ignorance (gnôsis versus agnoia).
Let us give the true reason. God is in no wise and in no manner unrighteous, but utterly and perfectly righteous, and there is nothing so like him as that one of us who in turn becomes most nearly perfect in righteousness. It is herein that the true cleverness of a man is found and also his worthlessness and cowardice; for the knowledge of this is wisdom or true virtue, and ignorance of it is folly or manifest wickedness; and all the other kinds of seeming cleverness and wisdom are paltry when they appear in public affairs and vulgar in the arts.
Sense-perception doesn’t attain to objective reality (ta koina), and thus can't be knowledge.
Your second question is answered through the idea of contemplation and rational thought as the highest virtue.
In Book VI of the Republic, he argues that the forms (higher realities and truths) can only be accessed through thought. For him, the highest of the three realities is the realm of the forms. It is timeless and immortal. It cannot be perceived by the senses and you may only access it through thinking philosophically and obtaining knowledge through more than your perceptions.
From Republic, Book VI: 510d-e
Then you also know that, although they use visible figures and make claims about them, their thought isn’t directed to them but to those other things that they are like. They make their claims for the sake of the square itself and the diagonal itself, not the diagonal they draw, and similarly with the others. These figures that they make and draw, of which shadows [e] and reflections in water are images, they now in turn use as images, in seeking to see those others themselves that one cannot see except by means of thought.
Also from the Republic Book VI:
Let us suppose that philosophical minds always love knowledge of a sort which shows them the eternal nature not varying from generation and corruption.